Infrared laser-imageable lithographic printing members and methods of preparing and imaging such printing members

ABSTRACT

Provided are methods of imaging a wet positive working lithographic printing member comprising the steps of providing a positive working printing member comprising a substrate, a hydrophilic layer, an infrared-absorbing layer, and, optionally, an ink-accepting surface layer; exposing the printing member to infrared layer imaging in an imagewise pattern removing by ablation not greater than 10% by weight, and most preferably none of the infrared-absorbing layer and optional ink-accepting surface layer; and removing with water the laser-exposed areas of the infrared-absorbing layer and optional ink-accepting surface layer to reveal the underlying hydrophilic layer. These methods are advantageous in reducing airborne debris and vapors during laser imaging, in increasing the speed of laser imaging, and in providing excellent cleanability and image quality. The printing member may further comprise a primer layer underlying the infrared-absorbing layer with an adhesion-promoting agent present in the primer layer. Also provided are methods of preparing a wet lithographic printing member and wet positive working lithographic printing members prepared according to the methods of this invention.

RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 09/410,230, filed Sep.30, 1999, and issued as U.S. Pat. No. 6,186,067 on Feb. 13, 2001.

FIELD OF THE INVENTION

The present invention relates in general to lithography and moreparticularly to systems for preparing lithographic printing plates usingdigitally controlled laser output. More specifically, this inventionrelates to methods for preparing a lithographic printing plateespecially suitable for directly imaging and utilizing with a wetlithographic printing press. The present invention also pertains to wetlithographic printing plates prepared according to such methods.

BACKGROUND OF THE INVENTION

Traditional techniques for introducing a printed image onto a recordingmaterial include letterpress printing and offset lithography. Both ofthese printing methods require a plate. To transfer ink in the patternof the image, the plate is usually loaded onto a plate cylinder of arotary press for efficiency. In letterpress printing, the image patternis represented on the plate in the form of raised areas that accept inkand transfer it onto the recording medium by impression. The term“lithographic,” as used herein, is meant to include various terms usedsynonymously, such as offset, offset lithographic, planographic, andothers. By the term “wet lithographic,” as used herein, is meant thetype of lithographic printing plate where the printing is based upon theimmiscibility of oil and water, wherein the oily material or ink ispreferentially retained by the image area and the water or fountainsolution is preferentially retained by the non-image area. When asuitably prepared surface is moistened with water and an ink is thenapplied, the background or non-image area retains the water and repelsthe ink while the image area accepts the ink and repels the water. Theink on the image area is then transferred to the surface of a materialupon which the image is to be reproduced, such as paper, cloth, and thelike. Commonly the ink is transferred to an intermediate material calledthe blanket, which in turn transfers the ink to the surface of thematerial upon which the image is to be reproduced. In a dry lithographicprinting system that does not utilize water, the plate is simply inkedand the image transferred directly onto a recording material ortransferred onto a blanket and then to the recording material.

Aluminum has been used for many years as a support for lithographicprinting plates. In order to prepare the aluminum for such use, it istypically subject to both a graining process and a subsequent anodizingprocess. The graining process serves to improve the adhesion of theimage to the plate and to enhance the water-receptive characteristics ofthe background areas of the printing plate. The graining and anodizingaffect both the performance and the durability of the printing plate.Both mechanical and electrolytic graining processes are well known andwidely used in the manufacture of lithographic printing plates.Processes for anodizing aluminum to form an anodic oxide coating andthen hydrophilizing the anodized surface by techniques such assilication are also well known in the art, and need not be furtherdescribed herein. The aluminum support is thus characterized by having aporous, wear-resistant hydrophilic surface, which specifically adapts itfor use in lithographic printing, particularly where long press runs arerequired.

The plates for a lithographic press are usually producedphotographically. The aluminum substrate described above is typicallycoated with a wide variety of photo-sensitive materials suitable forforming images for use in the lithographic printing process.Lithographic printing plates of this type are usually developed with anaqueous alkaline developing solution, which often additionally comprisesa substantial quantity of an organic solvent.

To prepare a wet plate using a typical negative-working subtractiveprocess, the original document is photographed to produce a photographicnegative. This negative is placed on an aluminum plate having awater-receptive oxide surface coated with a photopolymer. Upon exposureto light or other radiation through the negative, the areas of thecoating that received radiation (corresponding to the dark or printedareas of the original) cure to a durable oleophilic state. The plate isthen subjected to a developing process that removes the uncured areas ofthe coating (i.e., those which did not receive radiation, correspondingto the non-image or background areas of the original), thereby exposingthe hydrophilic surface of the aluminum plate.

Throughout this application, various publications, patents, andpublished patent applications are referred to by an identifyingcitation. The disclosures of the publications, patents, and publishedpatent applications referenced in this application are herebyincorporated by reference into the present disclosure to more fullydescribe the state of the art to which this invention pertains.

As is evident from the above description, photographic platemakingprocesses tend to be time consuming and require facilities and equipmentadequate to support the necessary chemistry. Efforts have been made formany years to manufacture a printing plate, which does not requireaqueous alkaline or solvent development or which only uses water fordevelopment. In addition, practitioners have developed a number ofelectronic alternatives to plate imaging, some of which can be utilizedon-press. With these systems, digitally controlled devices alter theink-receptivity of blank plates in a pattern representative of the imageto be printed. Such imaging devices include sources of electromagneticradiation, produced by one or more laser or non-laser sources, thatcreate physical and/or chemical changes on plate blanks; ink jetequipment that directly deposits ink-repellent or ink-accepting spots onplate blanks; and spark-discharge equipment, in which an electrode incontact with or spaced closely to a plate blank produces electricalsparks to physically alter the topology of the plate blank, therebyproducing “dots” which collectively form a desired image as for example,described in U.S. Pat. No. 4,911,075. Because of the ready availabilityof laser equipment and its amenability to digital control, significanteffort has been devoted to the development of laser-based imagingsystems.

In one such system, argon-ion, frequency-doubled Nd-YAG, and otherinfrared lasers are used to expose photosensitive blanks for traditionalchemical processing, as for example described in U.S. Pat. Nos.3,506,779; 4,020,762; 4,868,092; 5,153,236; 5,372,915; and 5,629,354. Inan alternative to this approach, a laser has been employed toselectively remove, in an imagewise pattern, an opaque coating thatoverlies a photosensitive plate blank. The plate is then exposed to asource of radiation, with the unremoved material acting as a mask thatprevents radiation from reaching underlying portions of the plate, asfor example described in U.S. Pat No. 4,132,168. However, the need forhigh writing speeds, coupled with the constraint of the low-poweredlasers favored by industry, has resulted in a requirement for printingplates that have a very high photosensitivity. Unfortunately, highphotosensitivity almost always reduces the shelf life of these plates.

Another approach to laser imaging uses thermal-transfer materials, asfor example described in U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389;4,395,946; and 5,395,729. With these systems, a polymer sheettransparent to the radiation emitted by the laser is coated with atransferable material. The transfer side of this construction is broughtinto contact with an acceptor sheet, and the transfer material isselectively irradiated through the transparent layer. Irradiation causesthe transfer material to adhere preferentially to the acceptor sheet.The transfer and acceptor materials exhibit different affinities forfountain solution and/or ink, so that removal of the transparent polymersheet with the unirradiated transfer material still on it leaves asuitably imaged, finished plate. Typically, the transfer material isoleophilic, and the acceptor material is hydrophilic. Plates producedwith transfer type systems tend to exhibit short useful lifetimes due tothe limited amount of material that can effectively be transferred:Airborne dirt can create an image quality problem depending on theparticular construction. In addition, because the transfer processinvolves melting and resolidification of material, image quality furthertends to be visibly poorer than that obtainable with other methods.

Other patents describe lithographic printing plates comprising a supportand a hydrophilic imaging layer which, upon imagewise laser exposure,becomes oleophilic in the exposed areas while remaining hydrophilic inthe unexposed areas, as for example disclosed in U.S. Pat. Nos.3,793,033, 4,034,183; 4,081,572; and 4,693,958. However, these types oflithographic printing plates suffer from the lack of a sufficient degreeof discrimination between oleophilic image areas and hydrophilicnon-image areas, with the result that image quality on printing is poor.

Early examples utilizing lasers used the laser to etch away materialfrom a plate blank to form an intaglio or letterpress pattern, as forexample described in U.S. Pat. Nos. 3,506,779 and 4,347,785. Thisapproach was later extended to production of lithographic plates, e.g.,by removal of a hydrophilic surface to reveal an oleophobic underlayer,as for example described in U.S. Pat. No. 4,054,094. These early systemsgenerally required high-power lasers, which are expensive and slow.

Other infrared laser ablation-based systems for imaging lithographicplates have been developed. These operate by laser-induced ablativeremoval of organic coating layers, which are coated onto a substratesuch as a polyester/metal laminate or onto a polymer coating on a metalsupport. Use of these polyester or polymer coating materials between theablation coating and the heat absorbing metal support provides a thermalbarrier material which reduces the amount of laser energy required toablate or fully remove the ablative-absorbing layer and any overlyingsurface layer, as for example described in Canadian Pat. No. 1,050,805and in U.S. Pat. Nos. 5,339,737; and 5,353,705. The laser exposure thusremoves one or more plate layers, resulting in an imagewise pattern offeatures on the plate. When the layers removed by laser ablation are theimage regions that accept ink, the plates are negative working. Whenlasers with a large spot size are used for imaging a negative workingplate, the size of the smallest printed dot is about as large as thespot size. Consequently, the image quality on printing may not be high.For example, a 35 micron laser spot size would print its smallest dotsize at about 35 microns with a negative working plate. On a 200 linesper inch (lpi) halftone screen, this is equivalent to a 5% to 6% dot.

U.S. Pat. No. 5,353,705 discloses a basic plate construction of alithographic plate having a secondary ablation layer intermediatebetween a substrate and a surface layer, such as a hydrophilic metalsubstrate and a radiation-absorptive and ablatively absorbing surfacelayer. The secondary ablation layer performs the protective or thermalbarrier function that shields the substrate from the thermal effects ofimaging radiation. The secondary ablation or thermal barrier layer ofthe '705 patent is ablated only partially in response to ablation of theablative-absorbing layer, is preferably substantially transparent to thelaser radiation and thereby not characterized by ablative absorption ofimaging radiation, and differs from the surface layer in its affinityfor at least one printing fluid selected from the group consisting ofink and a fluid that repels ink, i.e., when the surface layer isink-receptive and/or not receptive to a fountain solution, the thermalbarrier layer is not ink-receptive and/or is receptive to a fountainsolution, respectively. When the basic plate construction described inthe '705 patent has an ink receptive surface layer, and the thermalbarrier or secondary ablation layer is receptive to a fountain solutionand thus is not ink receptive, a positive working, wet lithographicplate results since the portions not removed by ablation are the imageregions that accept ink. Suitable polymeric materials for the secondaryablation layer of the '705 patent include, but are not limited to,polymethyl methacrylates, cellulosic ethers and esters, polyesters, andpolyurethanes. Hexamethoxymethylmelamine with p-toluenesulfonic acid maybe added to these polymeric materials.

U.S. Pat. No. 5,493,971 describes an example of such a positive working,wet lithographic plate. Its plate construction includes a hydrophilicmetal substrate, a polymeric, hydrophilic protective or thermal barriercoating which also may serve as an adhesion-promoting primer, and anink-accepting oleophilic surface layer characterized by ablativeabsorption of imaging radiation. The imaging laser interacts with theablatable surface layer, causing ablation thereof. After laser ablationimaging which removes at least the surface layer and also at least someof the hydrophilic protective layer as shown in FIG. 2 of the '971patent, the plate is then cleaned with a suitable solvent, e.g., water,to remove portions of the hydrophilic protective layer still remainingin the laser-exposed areas. Since the hydrophilic protective layer ispartially ablated in the '971 patent, but is not characterized byablative absorption of imaging radiation, this hydrophilic protectivelayer must not absorb the laser imaging radiation. It is thus similar tothe secondary ablation layer of the '705 patent which is partiallyablated and may be substantially transparent to the laser imagingradiation and thus not characterized by ablative absorption of thesurface layer. In the '971 patent, depending on the solubilityproperties of the residual plug of the partially ablated hydrophilicprotective layer in the cleaning solvent, e.g., water, the cleaning stepreveals the hydrophilic protective coating at less than its originalthickness, or reveals the hydrophilic metal substrate in the areas wherethe hydrophilic protective coating is entirely removed by the cleaningstep. After cleaning, the plate behaves like a conventional positiveworking wet lithographic metal plate on the printing press.

However, adhesion of the remaining ink-accepting surface coating to thehydrophilic protective layer has proven a difficult problem to overcome.Loss of adhesion can result if the protective hydrophilic thermalbarrier layer in the image or printing areas of the plate is damaged ordegraded during the laser imaging and cleaning process of the '971patent. For example, too much solvent or solubilization action by thecleaning solution or the fountain solution on press may erode the wallsof the image areas, eliminating the underlying support provided by thehydrophilic barrier layer around the periphery of the image feature anddegrading small image elements. This is particularly problematical whenthe hydrophilic protective coating layer is partially ablated andprobably further removed by the cleaning step and the action of thefountain solution such that the original surface of this protectivecoating layer is removed. This fully exposes the interface between theink-accepting layer and the hydrophilic protective coating layer, aswell as some of the wall of the hydrophilic protective coating layer atthe edge of the image feature, to these wet cleaning and fountainsolutions. This may lead to a major loss of image quality. Small dotsand type may be removed during the cleaning step or early in the printrun. Efforts to improve the adhesion of the laser ablatable surfacecoating and/or its durability to permit longer printing runs typicallyleads to a significant increase in the laser energy required to imagethe plate. International Publication No. WO 99/37481 discloses novelpositive working, wet lithographic printing plates and methods forpreparing such lithographic printing plates, which overcome thisadhesion problem.

U.S. Pat. No. 5,605,780 describes a laser-ablatable lithographicprinting plate comprising an anodized aluminum support having thereon anoleophilic image-forming layer comprising an infrared-absorbing agentdispersed in a film-forming cyanoacrylate polymer binder. Thehydrophilic protective layer has been eliminated. The '780 patentdescribes low required laser energy, good ink receptivity, good adhesionto the support, and good wear characteristics. Print runs of more than8200 impressions are shown in the examples.

U.S. Pat. No. 5,339,737 and Reissue Pat. No. 35,512 describe a varietyof ablation-type lithographic plate configurations for use with laserdiode imaging apparatus. These configurations include an ablation layer,which volatilizes into gaseous and particulate debris in response toinfrared imaging radiation. As used herein, the term “ablation” refersto the volatilization of a layer or a material into gaseous andparticulate debris in response to imaging radiation, which ablationresults in a loss of mass or weight in the layer or material. Forexample, U.S. Pat. No. 5,493,971 describes a complete or 100% ablativeloss of the ablative layer during the laser ablation imaging process,and FIG. 3A of International Publication No. WO 99/37481 describes apartial ablative loss of about 50% or greater of the ablatable layerduring the laser ablation imaging process.

Lithographic printing members are now commonly imaged by lower-powerlaser ablation imaging mechanisms. A major problem with these infraredlaser ablation-based systems for imaging lithographic plates has beenenvironmental. Because these operate by laser-induced destruction orremoval of organic polymers and other organic or inorganic materialswhich are coated in one or more layers overlying a substrate, airbornedebris and vapors are produced during imaging which may be hazardous tothe laser equipment and to the personnel who operate the equipment.Expensive equipment is generally required to contain the debris and tocapture the gases.

Despite the many efforts directed to the development of a laserimageable wet lithographic printing plate, there still remains a needfor plates that require no alkaline or solvent developing solution, thatperform like a conventional lithographic printing plate on press, thatare sensitive to a broad spectrum of laser energy such as 700 nm to 1150nm, that provide a high resolution and durable image, and that do notproduce debris and vapor requiring expensive and complex containmentequipment.

SUMMARY OF THE INVENTION

One aspect of the present invention pertains to methods of imaging a wetlithographic printing member, which methods comprise the steps of (a)providing a positive working lithographic printing member, whichpositive working member comprises a substrate, a hydrophilic layeroverlying the substrate, and an ink-accepting and infrared-absorbingsurface layer overlying the hydrophilic layer; wherein the surface layeris characterized by absorption of infrared imaging radiation, by beingnot removable by cleaning with water or a cleaning solution prior to theabsorption of infrared imaging radiation, and by being adapted to form awet lithographic printing surface as a result of an imagewise exposureto absorbable infrared radiation and subsequent removal of the exposedareas of the surface layer by cleaning with water or the cleaningsolution to reveal the underlying hydrophilic layer; and the hydrophiliclayer is characterized by being not removable by cleaning with water orthe cleaning solution; (b) exposing the positive working member of step(a) to absorbable infrared radiation using an infrared-emitting laser toeffect absorption of infrared radiation in the laser-exposed areas ofthe surface layer that is sufficient to cause the surface layer in thelaser-exposed areas to become removable by cleaning with water or thecleaning solution but insufficient to remove by ablation greater than10% by weight of the surface layer material in the laser-exposed areas;and (c) removing, with water or the cleaning solution, the laser-exposedareas of the surface layer to reveal the underlying hydrophilic layer.In one embodiment of the methods, the hydrophilic layer is characterizedby the absence of removal of the hydrophilic layer in the laser-exposedareas during steps (b) and (c). In a preferred embodiment, theabsorption of infrared radiation in the laser-exposed areas of thesurface layer of step (b) is sufficient to cause the surface layer inthe laser-exposed areas to become removable by cleaning with water orthe cleaning solution but insufficient to remove by ablation greaterthan 5% by weight, more preferably greater than 2% by weight, and mostpreferably none, of the surface layer in the laser-exposed areas. Thus,the methods of this invention provide a wet lithographic printing memberwith a very low or zero level of emission of gases and airborne debrisduring laser imaging, while also achieving excellent cleanability of thelaser-exposed areas by water or an equivalent environmentally-acceptableaqueous solution and excellent image resolution and durability due tothe properties of the infrared-absorbing and hydrophilic layers andtheir interface, as described herein.

Another aspect of the methods of imaging a wet lithographic printingmember of this invention pertains to the inclusion of a primer layerinterposed between the hydrophilic layer and the infrared-absorbinglayer to further enhance the desirable properties of theinfrared-absorbing and hydrophilic layers and their interface whichgreatly influence the amount of airborne materials produced during laserimaging, the speed of laser imaging, the ease of cleanability by waterduring removal of the laser-exposed areas, and the image resolution anddurability, as described herein. In one embodiment of the methods, aprimer layer is interposed between the hydrophilic layer and the surfacelayer in step (a) of the methods of this invention describedhereinabove, which primer layer comprises an adhesion-promoting agent.In one embodiment, the thickness of the primer layer of step (a) is from0.01 to 0.1 microns. In one embodiment, the adhesion-promoting agent ofthe primer layer comprises a crosslinked, polymeric reaction product ofa hydrophilic polymer and a crosslinking agent, and preferably, furthercomprises a catalyst. In one embodiment, the primer layer comprises anorganic sulfonic acid component. In one embodiment, the primer layercomprises a zirconium compound.

The term “printing member,” as used herein, is synonymous with the term“plate” and pertains to any type of printing member or surface capableof recording an image defined by regions exhibiting differentialaffinities for ink and/or fountain solution. The term “cleaningsolution,” as used herein, pertains to a solution used to clean orremove the coating or coatings from the laser-exposed regions of theprint members of the methods of this invention and may be water,combinations of at least 90% water and 10% or less organic solvents andadditives such as alcohols, surfactants, and glycols, and buffered orsalt-containing neutral or nearly neutral water solutions, such as knownin the art of aqueous fountain solutions for wet lithographic printing.The term “cleaning solution,” as used herein, does not include alkalineaqueous solutions with a pH of greater than about 10, acidic aqueoussolutions with a pH of less than about 3.5, or organic solvents withoutat least 90% by weight of water present. In a preferred embodiment, thesurface layer in the areas not exposed by the laser is furthercharacterized by being not removable by cleaning with water or acleaning solution and by durability on a wet lithographic printingpress.

In one embodiment, the weight of the infrared-absorbing layer of theprinting members of the methods of this invention is from about 0.05 toabout 1.0 g/m². In a preferred embodiment, the weight of theinfrared-absorbing layer is from about 0.1 to about 0.5/m².

In one embodiment of the methods of this invention, theinfrared-absorbing layer, which is the surface layer in a two layerplate construction of a hydrophilic layer and an infrared-absorbinglayer on a substrate, comprises one or more polymers and aninfrared-absorbing sensitizer. In one embodiment, the infrared-absorbingsensitizer is a carbon black. In one embodiment, the infrared-absorbinglayer comprises one or more carbon blacks selected from the groupconsisting of: sulfonated carbon blacks having sulfonated groups on thesurface of the carbon black, carboxylated carbon blacks having carboxylgroups on the surface of the carbon black, and carbon blacks having asurface active hydrogen content of not less than 1.5 mmol/g. In apreferred embodiment, the infrared-absorbing sensitizer is CAB-O-JET200. In another preferred embodiment, the infrared-absorbing sensitizeris BONJET BLACK CW-1. In one embodiment, the infrared-absorbingsensitizer is present in an amount greater than 55% by weight of theinfrared-absorbing layer. In one embodiment, the infrared-absorbingsensitizer is present in an amount greater than 65% by weight of theinfrared-absorbing layer.

In one embodiment of the methods of this invention, one of the one ormore polymers of the infrared-absorbing layer comprises a polymerselected from the group consisting of: polyvinyl alcohols,polyurethanes, epoxy polymers, vinyl polymers, acrylic polymers, andcellulosics. In one embodiment, the infrared-absorbing layer comprises apolyvinyl alcohol. In one embodiment, the polyvinyl alcohol is presentin an amount of 20 to 95 per cent by weight of the total weight ofpolymers present in the infrared-absorbing layer. As used herein, forthe purposes of determining the weight per cent of a material, the term“polymers” includes all the materials which are polymeric film formers,including monomeric species which polymerize or combine with a polymericspecies, such as, for example, a monomeric crosslinking agent. In oneembodiment, the polyvinyl alcohol is present in an amount of 25 to 75per cent by weight of the total weight of polymers present in theinfrared-absorbing layer. Suitable polymers for use in combination withpolyvinyl alcohol in the infrared-absorbing layer include, but are notlimited to other water-soluble or water-dispersible polymers such as,for example, polyurethanes, cellulosics, epoxy polymers, acrylicpolymers, and vinyl polymers.

In one embodiment of the methods of the present invention, theinfrared-absorbing layer comprises a crosslinking agent, preferably amelamine. In one embodiment, one or more polymers of theinfrared-absorbing layer comprise a crosslinked, polymeric reactionproduct of a polymer and a crosslinking agent. In a preferredembodiment, the crosslinked, polymeric reaction product is selected fromthe group consisting of crosslinked reaction products of a crosslinkingagent with the following polymers: a polyvinyl alcohol; a polyvinylalcohol and a vinyl polymer; a cellulosic polymer; a polyurethane; anepoxy polymer; an acrylic polymer; and a vinyl polymer.

In one embodiment of the methods of the present invention, theinfrared-absorbing layer further comprises a catalyst in addition to oneor more polymers and an infrared-absorbing sensitizer.

In one embodiment of the methods of this invention, theinfrared-absorbing layer comprises one or more polymers, aninfrared-absorbing sensitizer, and an organic sulfonic acid component,preferably a component of an amine-blocked p-toluenesulfonic acid. Inone embodiment, the organic sulfonic acid component is present in anamount of 25 to 75 weight per cent of the total weight of polymerspresent in the infrared-absorbing layer of the printing members of themethods of the present invention. In another embodiment, the organicsulfonic acid component is present in an amount of 35 to 55 weight percent of the total weight of polymers present in the infrared-absorbinglayer. In one embodiment, the infrared-absorbing layer comprises greaterthan 5% by weight of the organic sulfonic acid component. In oneembodiment, the infrared-absorbing layer comprises greater than 12% byweight of the organic sulfonic acid component.

In one embodiment of the methods of preparing a wet lithographicprinting member of the present invention, the hydrophilic layercomprises a crosslinked, polymeric reaction product of a hydrophilicpolymer and a first crosslinking agent. Suitable hydrophilic polymersfor the crosslinked, polymeric reaction product include, but are notlimited to, polyvinyl alcohols and cellulosics. In a preferredembodiment, the hydrophilic polymer is a polyvinyl alcohol. In oneembodiment, the first crosslinking agent is a zirconium compound. In oneembodiment, the first crosslinking agent is ammonium zirconyl carbonate.In a preferred embodiment, the first crosslinking agent is ammoniumzirconyl carbonate, and the ammonium zirconyl carbonate is present in anamount greater than 10% by weight of the polyvinyl alcohol, and, morepreferably, present in an amount of 20 to 50% by weight of the polyvinylalcohol. In another preferred embodiment, the hydrophilic layer furthercomprises a second crosslinking agent. In one embodiment, thehydrophilic layer further comprises a crosslinked, polymeric reactionproduct of a polyvinyl alcohol and the second crosslinking agent. In oneembodiment, the second crosslinking agent is a melamine. In oneembodiment, the hydrophilic layer further comprises a catalyst for thesecond crosslinking agent. In one embodiment, the catalyst is an organicsulfonic acid component. In one embodiment, the hydrophilic layercomprises an inorganic xerogel layer, which xerogel layer preferablycomprises a zirconium oxide xerogel.

In one embodiment of the printing members of the methods of the presentinvention, the thickness of the hydrophilic layer is from about 1 toabout 40 microns. In one embodiment, the thickness of the hydrophiliclayer is from about 2 to about 25 microns.

In one embodiment of the printing members of the methods of preparing awet lithographic printing plate member of this invention, suitablesubstrates comprise non-metal substrates and non-hydrophilic substrates,preferably papers, polymeric films, and non-hydrophilic metals such asnon-hydrophilic aluminum. In one embodiment, the substrate is selectedfrom the group of polymeric films consisting of: polyesters,polycarbonates, and polystyrene. In one embodiment, the polyesterpolymeric film is a polyethylene terephthalate film. In one embodiment,the non-hydrophilic metal substrate comprises a non-hydrophilic polymerlayer on at least one surface of the non-hydrophilic metal substrate. Inone embodiment, the substrate is a hydrophilic metal. Suitable metalsfor the hydrophilic metal substrate include, but are not limited to,aluminum, copper, steel, and chromium. In a preferred embodiment, themetal substrate is grained, anodized, silicated, or a combinationthereof. In one embodiment, the metal substrate is aluminum. In apreferred embodiment, the metal substrate is an aluminum substratecomprising a surface of uniform, non-directional roughness andmicroscopic depressions, which surface is in contact to the hydrophiliclayer and, more preferably, this surface of the aluminum substrate has apeak count in the range of 300 to 450 peaks per linear inch which extendabove and below a total bandwidth of 20 microinches.

Another aspect of the present invention pertains to methods of preparinga wet lithographic printing member, which methods comprise the steps of(a) coating onto a substrate a liquid mixture comprising a first liquidmedium, a hydrophilic polymer, and a first crosslinking agent; (b)drying the layer formed in step (a) to remove the first liquid medium,to cause a portion of the first crosslinking agent present to react, andto form a hydrophilic layer; (c) coating onto the hydrophilic layer aliquid mixture comprising a second liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent; (d)drying the layer formed in step (c) to remove the second liquid medium,to cause an additional portion of the first crosslinking agent presentin the hydrophilic layer to react, to cause a portion of the secondcrosslinking agent present to react, and to form an ink-accepting andinfrared-absorbing surface layer; thereby forming a positive workinglithographic printing member, wherein the surface layer and thehydrophilic layer are characterized by being not removable by cleaningwith water or a cleaning solution; (e) exposing the positive workingmember of step (d) to absorbable infrared radiation using aninfrared-emitting laser to effect absorption of infrared radiation inthe laser-exposed areas of the surface layer that is sufficient to causethe surface layer in the laser-exposed areas to become removable bycleaning with water or the cleaning solution but insufficient to removeby ablation greater than 10% by weight of the surface layer in thelaser-exposed areas; and (f) removing, with water or the cleaningsolution, the laser-exposed areas of the surface layer to reveal theunderlying hydrophilic layer. In one embodiment of the methods,subsequent to step (b) and prior to step (c), there are two steps of (i)coating onto the hydrophilic layer a liquid mixture comprising a liquidmedium and an adhesion-promoting agent; and (ii) drying the layer formedin step (i) to remove the liquid medium of step (i) and to form a primerlayer; and step (c) then comprises coating onto the primer layer theliquid mixture comprising a second liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent. In oneembodiment of the methods, the hydrophilic layer is characterized by theabsence of removal of the hydrophilic layer in the laser-exposed areasduring steps (e) and (f). In a preferred embodiment, the absorption ofinfrared radiation in the laser-exposed areas of step (e) is sufficientto cause the surface layer in the laser-exposed areas to becomeremovable by cleaning with water or the cleaning solution butinsufficient to remove by ablation greater than 5% by weight, morepreferably greater than 2% by weight, and most preferably none, of thesurface layer in the laser-exposed areas. In one embodiment, the weightof the infrared-absorbing layer of the printing members of the methodsof this invention is from about 0.05 to about 1.0 g/m². In a preferredembodiment, the weight of the infrared-absorbing layer is from about 0.1to about 0.5 g/m².

Another aspect of the present invention pertains to methods of preparinga wet lithographic printing member, which methods comprise the steps of:(a) coating onto a substrate a liquid mixture comprising a first liquidmedium, a hydrophilic polymer, and a first crosslinking agent; (b)drying the layer formed in step (a) to remove the first liquid mediumand to form a hydrophilic layer; (c) coating onto the hydrophilic layera liquid mixture comprising a second liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent; whereina portion of the second crosslinking agent penetrates into thehydrophilic layer; (d) drying the layer formed in step (c) and theunderlying hydrophilic layer to remove the second liquid medium, tocause a portion of the second crosslinking agent present in thehydrophilic layer to react, and to form an ink-accepting andinfrared-absorbing surface layer; thereby forming a positive workinglithographic printing member, wherein the surface layer and thehydrophilic layer are characterized by being not removable by cleaningwith water or a cleaning solution; (e) exposing the positive workingmember of step (d) to absorbable infrared radiation using aninfrared-emitting laser to effect absorption of infrared radiation inthe laser-exposed areas of the surface layer that is sufficient to causethe surface layer in the laser-exposed areas to become removable bycleaning with water or the cleaning solution but insufficient to removeby ablation greater than 10% by weight of the surface layer in thelaser-exposed areas; and (f) removing, with water or the cleaningsolution, the laser-exposed areas of the surface layer to reveal theunderlying hydrophilic layer. In one embodiment of the methods,subsequent to step (b) and prior to step (c), there are two steps of (i)coating onto the hydrophilic layer a liquid mixture comprising a liquidmedium and an adhesion-promoting agent; and (ii) drying the layer formedin step (i) to remove the liquid medium of step (i) and to form a primerlayer; and step (c) then comprises coating onto the primer layer theliquid mixture comprising a second liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent. In oneembodiment of the methods, the hydrophilic layer is characterized by theabsence of removal of the hydrophilic layer in the laser-exposed areasduring steps (e) and (f). In a preferred embodiment, the absorption ofinfrared radiation in the laser-exposed areas of the surface layer ofstep (e) is sufficient to cause the surface layer in the laser-exposedareas to become removable by cleaning with water or the cleaningsolution but insufficient to remove by ablation greater than 5% byweight, more preferably greater than 2% by weight, and most preferablynone, of the surface layer in the laser-exposed areas. In oneembodiment, the weight of the infrared-absorbing layer is from about0.05 to about 1.0 g/m². In a preferred embodiment, the weight of theinfrared-absorbing layer is from about 0.1 to about 0.5 g/m².

Still another aspect of this invention pertains to methods of preparinga wet lithographic printing member, which methods comprise the steps of:(a) coating onto a substrate a liquid mixture comprising a first liquidmedium, one or more hydrophilic polymers, and a first crosslinkingagent, wherein the first crosslinking agent is present in an amountgreater than 10% by weight of the one or more hydrophilic polymers; (b)drying the layer formed in step (a) to remove the first liquid mediumand to form a hydrophilic layer; (c) coating onto the hydrophilic layera liquid mixture comprising a second liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent; (d)drying the layer formed in step (c) to remove the second liquid mediumand to form an ink-accepting and infrared-absorbing surface layer,wherein the sensitizer is present in an amount of 25 to 80% by weight ofthe surface layer, and the one or more polymers are present in an amountof 10 to 60% by weight of the surface layer; thereby forming a positiveworking lithographic printing member, wherein the surface layer and thehydrophilic layer are characterized by being not removable by cleaningwith water or a cleaning solution; (e) exposing the positive workingmember of step (d) to absorbable infrared radiation using aninfrared-emitting laser to effect absorption of infrared radiation inthe laser-exposed areas of the surface layer that is sufficient to causethe surface layer in the laser-exposed areas to become removable bycleaning with water or the cleaning solution but insufficient to removeby ablation greater than 10% by weight of the surface layer in thelaser-exposed areas; and (f) removing, with water or the cleaningsolution, the laser-exposed areas of the surface layer to reveal theunderlying hydrophilic layer. In one embodiment of the methods, thehydrophilic layer is characterized by the absence of removal of thehydrophilic layer in the laser-exposed areas during steps (e) and (f).In a preferred embodiment, the absorption of infrared radiation in thelaser-exposed areas of the surface layer of step (b) is sufficient tocause the surface layer in the laser-exposed areas to become removableby cleaning with water or the cleaning solution but insufficient toremove by ablation greater than 5% by weight, more preferably greaterthan 2% by weight, and most preferably none, of the surface layer in thelaser-exposed areas. In one embodiment, the weight of theinfrared-absorbing surface layer is from about 0.05 to about 1.0 g/m².In one embodiment, the weight of the infrared-absorbing surface layer isfrom about 0.1 to about 0.5 g/m². Suitable polymers for forming theink-accepting and infrared-absorbing layer of steps (c) and (d) include,but are not limited to, polyvinyl alcohols, polyurethanes, epoxypolymers, vinyl polymers, acrylic polymers, and cellulosics.

Another aspect of the methods of imaging a wet lithographic printingmember of the present invention pertains to the inclusion of anadditional ink-accepting surface layer overlying the infrared-absorbinglayer to provide a basic three layer product design of ink-acceptingsurface layer/infrared-absorbing layer/hydrophilic layer on thesubstrate. This additional ink-accepting surface layer may be useful inachieving the best overall balance of properties, such as increasing thespeed of laser imaging, the ease of cleanability by water during removalof the laser-exposed areas, and particularly the image resolution anddurability. In one embodiment, the method of imaging a wet lithographicprinting member comprises the steps of (a) providing a positive workinglithographic printing member, which positive working member comprises asubstrate, a hydrophilic layer overlying the substrate, aninfrared-absorbing layer overlying the hydrophilic layer, and anink-accepting surface layer overlying the infrared-absorbing layer; thesurface layer being characterized by the absence of ablation fromabsorption of infrared imaging radiation; the infrared-absorbing layerbeing characterized by absorption of imaging radiation; the surfacelayer and the infrared-absorbing layer being characterized by being notremovable by cleaning with water or a cleaning solution prior to theabsorption of infrared imaging radiation and by being adapted to form awet lithographic printing surface as a result of an imagewise exposureto absorbable infrared radiation and subsequent removal of the exposedareas of the surface layer and the infrared-absorbing layer by cleaningwith water or the cleaning solution to reveal the underlying hydrophiliclayer; and the hydrophilic layer being characterized by being notremovable by cleaning with water or the cleaning solution; (b) exposingthe positive working member of step (a) to absorbable infrared radiationusing an infrared-emitting laser to effect absorption of infraredradiation in the laser-exposed areas of the infrared-absorbing layerthat is sufficient to cause the surface layer and the infrared-absorbinglayer in the laser-exposed areas to become removable by cleaning withwater or the cleaning solution but insufficient to remove by ablationgreater than 10% by weight of the combined surface layer andinfrared-absorbing layer in the laser-exposed areas; and (c) removing,with water or the cleaning solution, the laser-exposed areas of thesurface layer and the infrared-absorbing layer to reveal the underlyinghydrophilic layer.

Another aspect of the methods of imaging a wet lithographic printingmember with a basic three layer product design of this inventionpertains to the inclusion of a primer layer interposed between thehydrophilic layer and the infrared-absorbing layer to further enhancethe desirable properties of the infrared-absorbing and hydrophiliclayers and their interface which greatly influence the amount ofairborne materials produced during laser imaging, the speed of laserimaging, the ease of cleanability by water during removal of thelaser-exposed areas, and the image resolution and durability, asdescribed herein. In one embodiment of the methods, a primer layer isinterposed between the hydrophilic layer and the infrared-absorbinglayer in step (a) of the methods, which primer layer comprises anadhesion-promoting agent. In one embodiment, the thickness of the primerlayer of step (a) is from 0.01 to 0.1 microns. In one embodiment, theadhesion-promoting agent of the primer layer comprises a crosslinked,polymeric reaction product of a hydrophilic polymer and a crosslinkingagent, and preferably, further comprises a catalyst. In one embodiment,the primer layer comprises an organic sulfonic acid component. In oneembodiment, the primer layer comprises a zirconium compound.

In one embodiment of the methods of imaging a wet lithographic printingmember with a three-layer product design with optional primer layer ofthis invention, the ink-accepting surface layer comprises a crosslinked,polymeric reaction product of a polymer and a crosslinking agent.Suitable polymers for the crosslinked, polymeric reaction productincludes, but are not limited to, cellulosics, acrylic polymers,polyurethanes, and epoxy polymers. In one embodiment, the ink-acceptingsurface layer further comprises an organic sulfonic acid component. Inone embodiment, the weight of the ink-accepting surface layer is fromabout 0.05 to about 0.5 g/m². In a preferred embodiment, the weight ofthe ink-accepting surface layer is from about 0.1 to about 0.3 g/m². Inone embodiment of the methods, the hydrophilic layer is characterized bythe absence of removal of the hydrophilic layer in the laser-exposedareas during steps (b) and (c). In a preferred embodiment, theabsorption of infrared radiation in the laser-exposed areas of theinfrared-absorbing layer of step (b) is sufficient to cause theink-accepting surface layer and the infrared-absorbing layer in thelaser-exposed areas to become removable by cleaning with water or thecleaning solution but insufficient to remove by ablation greater than 5%by weight, more preferably greater than 2% by weight, and mostpreferably none, of the combined ink-accepting surface layer andinfrared-absorbing layer in the laser-exposed areas.

Another aspect of the present invention pertains to methods of preparinga wet lithographic printing member with a basic three layer productdesign, which methods comprise the steps of (a) coating onto a substratea liquid mixture comprising a first liquid medium, a hydrophilicpolymer, and a first crosslinking agent; (b) drying the layer formed instep (a) to remove the first liquid medium, to cause a portion of thefirst crosslinking agent present to react, and to form a hydrophiliclayer; (c) coating onto the hydrophilic layer a liquid mixturecomprising a second liquid medium, a polymer, an infrared-absorbingsensitizer, and a second crosslinking agent; (d) drying the layer formedin step (c) to remove the second liquid medium, to cause an additionalportion of the first crosslinking agent present in the hydrophilic layerto react, to cause a portion of the second crosslinking agent present toreact, and to form an infrared-absorbing layer; (e) coating onto theinfrared-absorbing layer a liquid mixture comprising a third liquidmedium and an ink-accepting polymer; (f) drying the layer formed in step(e) to remove the third liquid medium and to form an ink-acceptingsurface layer; thereby forming a positive working wet lithographicprinting member, wherein the surface layer, the infrared-absorbinglayer, and the hydrophilic layer are characterized by being notremovable by cleaning with water or a cleaning solution; (g) exposingthe positive working member of step (f) to absorbable infrared radiationusing an infrared-emitting laser to effect absorption of infraredradiation in the laser-exposed areas of the infrared-absorbing layerthat is sufficient to cause the surface layer and the infrared-absorbinglayer in the laser-exposed areas to become removable by cleaning withwater or the cleaning solution but insufficient to remove by ablationgreater than 10% by weight of the combined surface layer andinfrared-absorbing layer in the laser-exposed areas; and (h) removing,with water or the cleaning solution, the laser-exposed areas of thesurface and infrared-absorbing layers to reveal the underlyinghydrophilic layer. In one embodiment of the methods, subsequent to step(b) and prior to step (c), there are two steps of (i) coating onto thehydrophilic layer a liquid mixture comprising a liquid medium and anadhesion-promoting agent; and (ii) drying the layer formed in step (i)to remove the liquid medium of step (i) and to form a primer layer; andstep (c) then comprises coating onto the primer layer the liquid mixturecomprising a second liquid medium, a polymer, an infrared-absorbingsensitizer, and a second crosslinking agent.

Still another aspect of the methods of preparing a wet lithographicprinting member having a three layer product design of this inventionpertains to methods comprising the steps of (a) coating onto a substratea liquid mixture comprising a first liquid medium, a hydrophilicpolymer, and a first crosslinking agent; (b) drying the layer formed instep (a) to remove the first liquid medium and to form a hydrophiliclayer; (c) coating onto the hydrophilic layer a liquid mixturecomprising a second liquid medium, a polymer, an infrared-absorbingsensitizer, and a second crosslinking agent; wherein a portion of thesecond crosslinking agent penetrates into the hydrophilic layer; (d)drying the layer formed in step (c) and the underlying hydrophilic layerto remove the second liquid medium, to cause a portion of the secondcrosslinking agent present in the hydrophilic layer to react, and toform an infrared-absorbing layer; (e) coating onto theinfrared-absorbing layer a liquid mixture comprising a third liquidmedium and an ink-accepting polymer; (f) drying the layer formed in step(e) to remove the third liquid medium and to form an ink-acceptingsurface layer; thereby forming a positive working wet lithographicprinting member, wherein the surface layer, the infrared-absorbinglayer, and the hydrophilic layer are characterized by being notremovable by cleaning with water or a cleaning solution; (g) exposingthe positive working member of step (f) to absorbable infrared radiationusing an infrared-emitting laser to effect absorption of infraredradiation in the laser-exposed areas of the infrared-absorbing layerthat is sufficient to cause the surface and infrared-absorbing layers inthe laser-exposed areas to become removable by cleaning with water orthe cleaning solution but insufficient to remove by ablation greaterthan 10% by weight of the combined surface layer and infrared-absorbinglayer in the laser-exposed areas; and (h) removing, with water or thecleaning solution, the laser-exposed areas of the surface andinfrared-absorbing layers to reveal the underlying hydrophilic layer. Inone embodiment of the methods, subsequent to step (b) and prior to step(c), there are two steps of (i) coating onto the hydrophilic layer aliquid mixture comprising a liquid medium and an adhesion-promotingagent; and (ii) drying the layer formed in step (i) to remove the liquidmedium of step (i) and to form a primer layer; and step (c) thencomprises coating onto the primer layer the liquid mixture comprising asecond liquid medium, a polymer, an infrared-absorbing sensitizer, and asecond crosslinking agent.

In one embodiment, the infrared-absorbing layer of the three layerdesigns of the printing members of the methods of the present inventionis ink-accepting. In one embodiment, the infrared-absorbing layer of thethree layer designs of the printing members of the methods of thepresent invention is further characterized by not accepting ink and byaccepting water on a wet lithographic printing press.

Another aspect of this invention pertains to methods for preparing apositive working, wet lithographic printing member, for both two layerand three layer product designs with highly crosslinked layers and withvarious approaches for interaction of the crosslinking chemistry byinterfacial reactions between adjacent infrared-absorbing andhydrophilic layers. The infrared-absorbing sensitizers in theinfrared-absorbing layer for use with the highly crosslinked layers ofthe present invention are not limited to organic sensitizers, such ascarbon blacks and organic dyes, but may include inorganic and metallicsensitizers.

Still another aspect of the present invention pertains to wetlithographic printing members prepared according to the methods of thisinvention.

One advantage of the present invention is that the lithographic printingmember or plate may be imaged at very low laser power, which eliminatesablation of the infrared-absorbing layer and of the ink acceptingsurface layer, if present, thus eliminating almost all or all noxiousvapors and airborne debris. Since the water-based fountain solution onthe wet lithographic printing press will easily clean the laser-exposedinfrared-absorbing layer, and also the ink-accepting surface layer ifpresent, from the plate, the plate is suitable for on press imaging anddirect printing. Also, in the course of a long printing run, thehydrophilic layer is not solubilized by the fountain solution, andnon-hydrophilic substrates may be utilized. Further, the hydrophiliclayer under the non-exposed image areas of the present inventionprovides excellent adhesion to the overlying ink-accepting image layersince it is nearly impossible to undercut through solubilization,particularly when the hydrophilic layer is highly crosslinked, includingat its interface to the infrared-absorbing layer.

The superiority of the methods and of the lithographic printing membersof the methods of the present invention over those previously known isparticularly manifest in the ability to be imaged rapidly withrelatively inexpensive diode lasers with large spot sizes; its low laserpower imaging characteristic; its elimination of noxious vapors andairborne debris during imaging; its ease of cleaning; its excellentimage resolution and printing quality; its resistance to water, whichprovides excellent durability and image adhesion on the printing press;and its low cost of manufacture.

As one of skill in the art will appreciate, features of one embodimentand aspect of the invention are applicable to other embodiments andaspects of the invention The above-discussed and other features andadvantages of the present invention will be appreciated and understoodby those skilled in the art from the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention when taken inconjunction with the accompanying drawings.

FIG. 1 shows enlarged cross-sectional views of the mechanism, as knownin the art, for imaging and cleaning a wet lithographic plate having anabsorptive, ablatable top layer, a protective layer, and a hydrophilicmetal substrate.

FIG. 2 shows an enlarged cross-sectional view of a lithographic printingmember having an infrared-absorbing, ink-accepting surface layer, ahydrophilic layer, and a substrate.

FIGS. 3A and 3B show enlarged cross-sectional views of the lithographicprinting member of FIG. 2 in one embodiment of the methods of preparinga wet lithographic printing member of this invention: (A) after exposingto infrared laser imaging radiation; and (B) after cleaning.

FIG. 4 shows an enlarged cross-sectional view of a lithographic printingmember having an ink-accepting surface layer, an infrared-absorbinglayer, a hydrophilic layer, and a substrate.

FIGS. 5A and 5B show enlarged cross-sectional views of the lithographicprinting member of FIG. 4 in one embodiment of the methods of preparinga wet lithographic printing member of this invention: (A) after exposingto infrared laser imaging radiation; and (B) after cleaning.

DETAILED DESCRIPTION OF THE INVENTION

Methods of Preparing and Imaging Lithographic Printing Members Having ATwo Layer Product Configuration

In preparing lithographic printing members by laser imaging, it would behighly desirable to eliminate or at least greatly reduce the vapors andairborne debris generated in ablative laser imaging. For example, asshown in FIG. 1, U.S. Pat. No. 5,493,971 describes a laser ablationimaging process where imaging radiation fully removes surface layer 10and at least some of protective layer 12, leaving a residual plug of theprotective layer material. The presence of full ablation of at least onelayer during the laser imaging process results in airborne debris, andpossibly objectionable or noxious vapors and odors, which subsequentlyneed to be captured by expensive and possibly complex environmentalcontrol equipment in the laser imaging system. The present inventionovercomes this limitation of ablative laser imaging methods and systemsfor preparing lithographic printing members by providing laser imagingmethods and systems that comprise an absence of any ablation, or atleast almost all ablation, during the laser imaging exposure.

A two layer product configuration for the lithographic printing membersfor use in the methods of preparing a wet lithographic printing memberof the present invention is illustrated in FIG. 2. This two layerproduct configuration comprises an infrared-absorbing and ink-acceptingsurface layer 102 and a hydrophilic layer 104 on a substrate 106. FIGS.3A and 3B show one embodiment of the methods of preparing a wetlithographic printing member of this invention for the lithographicprinting member of FIG. 2. As shown in FIG. 3A, laser imaging radiationinteracts with layers 102 and 104 without generating ablationbyproducts, such as airborne debris and vapors, and leaves alaser-exposed area 108 on the hydrophilic layer 104. As shown in FIG.3B, the laser-imaged plate is then cleaned with a cleaning solution,such as water, in order to remove laser exposed area 108, therebyexposing the surface 110 of the hydrophilic layer 104. None of thishydrophilic layer 104 is ablated during the laser imaging steps, asillustrated in FIG. 3A. Also, none of this hydrophilic layer 104 isremoved during the cleaning step, as illustrated in FIG. 3B. Theablation-resistant and cleaning-resistant properties of this hydrophiliclayer, and the interfacial properties between the hydrophilic andinfrared-absorbing layers, are among the key features of this invention,as described herein.

Thus, one aspect of the present invention pertains to methods of imaginga wet lithographic printing member, which methods comprise the steps of(a) providing a positive working lithographic printing member, whichpositive working member comprises a substrate, a hydrophilic layeroverlying the substrate, and an ink-accepting and infrared-absorbingsurface layer overlying the hydrophilic layer; wherein the surface layeris characterized by absorption of infrared imaging radiation, by notbeing removable by cleaning with water or a cleaning solution prior tothe absorption of infrared imaging radiation, and by being adapted toform a wet lithographic printing surface as a result of an imagewiseexposure to absorbable infrared radiation and subsequent removal of theexposed areas of the surface layer by cleaning with water or thecleaning solution to reveal the underlying hydrophilic layer; and thehydrophilic layer is characterized by being not removable by cleaningwith water or the cleaning solution; (b) exposing the positive workingmember of step (a) to absorbable infrared radiation using an infraredemitting laser to effect absorption of infrared radiation in thelaser-exposed areas of the surface layer that is sufficient to cause thesurface layer in the laser-exposed areas to become removable by cleaningwith water or the cleaning solution but insufficient to remove byablation greater than 10% by weight of the surface layer in the laserexposed areas; and (c) removing, with water or the cleaning solution,the laser-exposed areas of the surface layer to reveal the underlyinghydrophilic layer. In one embodiment of the methods, the hydrophiliclayer is characterized by the absence of removal of the hydrophiliclayer in the laser-exposed areas during steps (b) and (c). In apreferred embodiment, the absorption of infrared radiation in thelaser-exposed areas of the surface layer of step (b) is sufficient tocause the surface layer in the laser-exposed areas to become removableby cleaning with water or the cleaning solution but insufficient toremove by ablation greater than 5% by weight, more preferably greaterthan 2% by weight, and most preferably none, of the infrared-absorbingor surface layer in the laser-exposed area. Thus, the methods of thisinvention provide a wet lithographic printing member with a very low orzero level of emission of gases and air-borne debris during laserimaging, while also achieving excellent cleanability of thelaser-exposed areas by water or an equivalent environmentally-acceptableaqueous solution and excellent image resolution and durability due tothe properties of the infrared-absorbing and hydrophilic layers andtheir interface, as described herein.

In one embodiment, the weight of the infrared-absorbing layer of theprinting members of the methods of this invention is from about 0.05 toabout 1.0 g/m². In a preferred embodiment, the weight of theinfrared-absorbing layer is from about 0.1 to about 0.5 g/m². Theseweights are similar to or lower than those typically used in theinfrared-absorbing layers of the ablative laser imageable lithographicprinting members as, for example, described in Can. Pat. No. 1,050,805and U.S. Pat. No. 5,493,971. Thus, in the embodiments of this inventionwhere a small amount of ablation of the infrared-absorbing layers, suchas 10% or less by weight, occurs during laser imaging, the amount ofablative byproducts from these thin infrared-absorbing layers,particularly those less than 0.5 g/m² in weight, is extremely small anddoes not represent a significant environmental containment issue incomparison to ablative laser imaging methods involving full ablation ofthicker infrared-absorbing layers.

Hydrophilic Layers for Methods of Preparing and Imaging LithographicPrinting Members

As, for example, illustrated in FIGS. 2, 3A, and 3B, the hydrophiliclayer 104 provides a thermal barrier during laser exposure to preventheat loss and possible damage to the substrate 106, particularly whenthe substrate is a metal, such as aluminum. Also, it is hydrophilic sothat it may function as the background hydrophilic or water-loving areaon the imaged wet lithographic plate.

The hydrophilic layer 104 should adhere well to the substrate 106 and tothe infrared-absorbing layer 102. In general, polymeric materialssatisfying these criteria include those having exposed polar moietiessuch as hydroxyl or carboxyl groups such as, for example, variouscellulosics modified to incorporate such groups, and polyvinyl alcoholpolymers.

The hydrophilic third layer 104 withstands repeated application offountain solution during printing without substantial degradation orsolubilization. One test of withstanding the repeated application offountain solution during printing is a wet rub resistance test, asdescribed in Example 2 of this invention. Satisfactory results fordurability in withstanding the repeated application of fountain solutionand not being soluble in water, as defined herein for the presentinvention, are the retention of the 3% dots in the wet rub resistancetest.

Suitable polyvinyl alcohol-based coatings may be obtained by combining,for example, AIRVOL 125 polyvinyl alcohol, a trademark for polyvinylalcohol polymers available from Air Products and Chemicals, Inc.Allentown, Pa.; BACOTE 20, a trademark for an ammonium zirconylcarbonate solution available from Magnesium Elecktron, Flemington, N.J.;glycerol, available from Aldrich Chemical, Milwaukee, Wis.; and TRITONX-100, a trademark for a surfactant available from Rohm & Haas,Philadelphia, Pa.

In addition to its ability to crosslink polymers having hydroxyl andcarboxyl groups, ammonium zirconyl carbonate is known to be a precursor,upon heating, to the formation of colloidal zirconium oxide sols andzirconium oxide sol-gel or xerogel layers. In the hydrophilic layers ofthe lithographic printing members of the methods of this invention,incorporating a large amount of a sol-gel or xerogel precursor, such asammonium zirconyl carbonate, is advantageous in providing a combinationof durability against removal by ablation during laser imaging andagainst removal by cleaning with water or a cleaning solution togetherwith excellent adhesion to the substrate and the overlying coatinglayer, hydrophilic properties, and ease of cleanability of thenon-ablated infrared-absorbing layer from the surface of the hydrophiliclayer in the laser-exposed areas, as illustrated in FIG. 3B. Besidesutilizing a first crosslinking agent, such as ammonium zirconylcarbonate, in the hydrophilic layer, and preferably, utilizing a xerogelprecursor, such as ammonium zirconyl carbonate in an amount greater thanrequired to crosslink the available crosslinkable groups on the polymerspresent, it is preferred to add a second crosslinking agent, such as amelamine, to further crosslink the hydrophilic layer and to improve theease of cleanability at the interface between the hydrophilic layer andthe infrared-absorbing layer. A catalyst, such as an organic sulfonicacid component as defined herein, may be added to the hydrophilic layerto increase the rate of reaction of the first and/or second crosslinkingagents. The second crosslinking agent and the catalyst may be in thecoating formulation of the hydrophilic layer or may be in the coatingformulation of an overlying layer, such as the infrared-absorbing layer.In the latter case, the second crosslinking agent and the catalyst areintroduced into the hydrophilic layer by penetration of the liquidcoating mixture of the overlying layer into the hydrophilic layer. Inthis case, it is usually important to control the degree of cure of thehydrophilic layer before application of the overlying layer in order toachieve the optimal balance of desired properties, such as ease ofcleanability and image resolution and durability. Additional curing ofthe hydrophilic layer with the first and second crosslinking agentspresent may then be accomplished by heating during the drying of theoverlying layer and/or during any subsequent post-heating steps.

Thus, in one embodiment of the methods of preparing a wet lithographicprinting member of the present invention, the hydrophilic layercomprises a crosslinked, polymeric reaction product of a hydrophilicpolymer and a first crosslinking agent. Suitable hydrophilic polymersfor the crosslinked, polymeric reaction products include, but are notlimited to, polyvinyl alcohols and cellulosics. In a preferredembodiment, the hydrophilic polymer is a polyvinyl alcohol. In oneembodiment, the first crosslinking agent is a zirconium compound. In oneembodiment, the first crosslinking agent is ammonium zirconyl carbonate.In a preferred embodiment, the first crosslinking agent is ammoniumzirconyl carbonate, and the ammonium zirconyl carbonate is present in anamount greater than 10% by weight of the polyvinyl alcohol, and, morepreferably, present in an amount of 20 to 50% by weight of the polyvinylalcohol. In another preferred embodiment, the hydrophilic layer furthercomprises a second crosslinking agent. In one embodiment, thehydrophilic layer further comprises a crosslinked, polymeric reactionproduct of a polyvinyl alcohol and a second crosslinking agent. In oneembodiment, the second crosslinking agent is a melamine. In oneembodiment, the hydrophilic layer further comprises a catalyst for thesecond crosslinking agent. In one embodiment, the catalyst is an organicsulfonic acid component. In one embodiment, the hydrophilic layercomprises an inorganic xerogel layer, which layer preferably comprises azirconium oxide xerogel.

The hydrophilic layer is coated in this invention typically at athickness in the range of from about 1 to about 40 microns and morepreferably in the range of from about 2 to about 25 microns. Aftercoating, the hydrophilic layer may be dried at a variety of temperaturesin order to remove the volatile liquid from the coating mixture. Thetemperature conditions for curing of the hydrophilic layer depend onmany factors, such as on the particular materials in the layer and onwhether a crosslinking agent and/or catalyst from a subsequent coatingapplication needs to penetrate and further cure the hydrophilic layer.For example, in the case where a second crosslinking agent and acatalyst from a subsequent coating application will penetrate andfurther cure the hydrophilic layer, the hydrophilic layer may be driedat a temperature of 135° C. to 155° C. and subsequently only partiallycured at a temperature between 145° C. to 185° C. for between 10 secondsand 3 minutes and more preferably at a temperature between 145° C. and165° C. for between 30 seconds and 2 minutes.

Infrared-Absorbing Layers for Methods of Preparing and ImagingLithographic Printing Members

The primary characteristics of the infrared-absorbing layer areabsorption of and sensitivity to infrared laser imaging radiation usingcommercially practicable laser imaging equipment, sufficient adhesion tothe hydrophilic layer to provide long running plates, and retention ofsmall 150 lpi 2% and 3% dots in halftone images when running on press.Also, the infrared-absorbing layer is characterized by being notremovable by cleaning with water or a cleaning solution prior to theabsorption of infrared imaging radiation. It is also preferable that theinfrared-absorbing layer image at low imaging laser power levels whereablation of the infrared-absorbing layer does not take place, yet thelaser-exposed areas of the infrared-absorbing layer can be subsequentlyeasily removed by cleaning with water or a cleaning solution. Adhesionto the hydrophilic layer is dependent in part upon the chemicalstructure and the amount of the material that absorbs the laser imagingradiation and the bonding sites available on the polymers in theinfrared-absorbing layer.

It is important that the bonding by the polymers and other materials inthe infrared-absorbing layer is strong enough to provide adequateadhesion to the hydrophilic layer, but is chemically transformed duringlaser exposure sufficiently that it subsequently provides ease ofcleaning of the laser-exposed areas of the infrared-absorbing layer fromthe hydrophilic layer. For example, vinyl-type polymers, such aspolyvinyl alcohol, strike an appropriate balance between these twoproperties. For example, improved adhesion to the hydrophilic layer aswell as easy cleaning after imaging is provided by use of a polyvinylalcohol, such as AIRVOL 125 and AIRVOL 325, incorporated into theinfrared-absorbing layer. Crosslinking agents may also be added. Aninfrared radiation-absorbing compound or sensitizer is added to thecomposition of the infrared-absorbing layer and dispersed therein. Avariety of infrared-absorbing compounds are known and may be utilized asthe radiation-absorbing sensitizer in the present invention. Of theinfrared sensitizers evaluated, CAB-O-JET 200, a tradename for surfacemodified carbon blacks available from Cabot Corporation, Bedford, Mass.,and BONJET BLACK CW-1, a trademark for a surface modified carbon blackaqueous dispersion available from Orient Corporation, Springfield, N.J.,surprisingly least affected the adhesion to the hydrophilic layer at theamounts required to give adequate sensitivity for imaging. Thus forexample, BONJET BLACK CW-1 has good infrared-sensitizing properties,allows enhanced adhesion to the hydrophilic layer, and also surprisinglyallows for easily cleaning and removing the laser-imaged areas of theinfrared-absorbing layer after laser exposure under the non-ablativelaser imaging conditions of this invention.

Suitable coatings for both the hydrophilic and infrared layers may beprepared by known mixing methods. For example, for theinfrared-absorbing layer, a base coating mix may be formed by firstmixing all the components, such as water; 2-butoxyethanol; AIRVOL 125polyvinyl alcohol; CYMEL 303 hexamethoxymethylmelamine crosslinkingagent; WITCOBOND 240, a trademark for a water based polyurethanedispersion available from Witco Corporation; and BONJET BLACK CW-1carbon black, except for not including any crosslinking catalyst. Toextend the stability of the coating formulation, any crosslinking agent,such as NACURE 2530, may be subsequently added to the base coating mixor dispersion just prior to the coating application. The coating mix ordispersion may be applied by any of the known methods of coating.However, to best achieve the benefits of the invention, the applicationof the infrared-absorbing coating layer overlying the hydrophilic layermust preferably be done in a controlled manner. For example, the amountand percent solids of coating applied and the wet dwell time beforedrying of the coating should be closely controlled when application isover a partially cured hydrophilic coating. Typically, better resultsare achieved when the hydrophilic coating layer is not wet by largeamounts of the infrared-absorbing coating mix and when the wet dwelltime is maintained between 1 to 12 seconds, preferably between 1 to 6seconds. A variety of application methods, such as, for example, wirewound rod coating, reverse roll coating, gravure coating, squeezecoating and slot die coating may all be utilized. After drying to removethe volatile liquids and to provide the level of curing desired, a solidcoating layer is formed.

In one embodiment, the weight of the infrared-absorbing layer is fromabout 0.05 to about 1.0 g/m². In a preferred embodiment, the weight ofthe infrared-absorbing layer is from about 0.1 to about 0.5 g/m². Avariety of drying conditions may be used to remove the volatile liquidsfrom the infrared-absorbing layer, to cure the infrared-absorbing layer,and to further cure the hydrophilic layer, if desired. For example,after coating, the layer may be dried and subsequently cured at atemperature between 135° C. and 185° C. for between 10 seconds and 3minutes and more preferably cured at a temperature between 145° C. and175° C. for between 30 seconds to 2 minutes.

In one embodiment, the infrared-absorbing layer of the printing memberof the methods of the present invention is ink-accepting. Anink-accepting, infrared-absorbing layer of this invention isillustrated, for example, as layer 102 in FIGS. 2, 3A, and 3B, and inExample 2.

In another embodiment, the infrared-absorbing layer may be characterizedby not accepting ink and by accepting water on a wet lithographicprinting press. For use in positive working, wet lithographic printingmember applications, an ink-accepting topcoat layer is utilized withthis construction.

In one embodiment of the methods of this invention, the infraredabsorbing layer, which is the surface layer in a two layer plateconstruction of a hydrophilic layer and an infrared absorbing layer on asubstrate, comprises one or more polymers and an infrared-absorbingsensitizer. In one embodiment, the infrared-absorbing sensitizer is acarbon black. In one embodiment, the infrared-absorbing layer comprisesone or more carbon blacks selected from the group consisting of:sulfonated carbon blacks having sulfonated groups on the surface of thecarbon black, carboxylated carbon blacks having carboxyl groups on thesurface of the carbon black, and carbon blacks having a surface activehydrogen content of not less than 1.5 mmol/g. Carbon blacks having asurface active hydrogen content of not less than 1.5 mmol/g aredescribed, for example, in U.S. Pat. No. 5,609,671. In a preferredembodiment, the infrared-absorbing sensitizer is CAB-O-JET 200. Inanother preferred embodiment, the infrared-absorbing sensitizer isBONJET BLACK CW-1. In one embodiment, the infrared-absorbing sensitizeris present in an amount greater than 55% by weight of theinfrared-absorbing layer. In one embodiment, the infrared-absorbingsensitizer is present in an amount greater than 65% by weight of theinfrared-absorbing layer.

In one embodiment of the methods of this invention, one of the one ormore polymers of the infrared-absorbing layer comprises a polymerselected from the group consisting of: polyvinyl alcohols,polyurethanes, epoxy polymers, vinyl polymers, acrylic polymers, andcellulosics. As used herein, the term “acrylic polymers” pertains topolymers that comprise acrylic acid, acrylate, or methacrylate groups.In one embodiment, the infrared-absorbing layer comprises a polyvinylalcohol. In one embodiment, the polyvinyl alcohol is present in anamount of 20 to 95 per cent by weight of the total weight of polymerspresent in the infrared-absorbing layer. In one embodiment, thepolyvinyl alcohol is present in an amount of 25 to 75 per cent by weightof the total weight of polymers present in the infrared-absorbing layer.Suitable polymers for use in combination with polyvinyl alcohol in theinfrared-absorbing layer include, but are not limited to otherwater-soluble or water-dispersible polymers such as, for example,polyurethanes, cellulosics, epoxy polymers, acrylic polymers, and vinylpolymers.

In one embodiment of the methods of the present invention, theinfrared-absorbing layer comprises a crosslinking agent, preferably amelamine. In one embodiment, one or more polymers of theinfrared-absorbing layer comprise a crosslinked, polymeric reactionproduct of a polymer and a crosslinking agent. In a preferredembodiment, the crosslinked, polymeric reaction product is selected fromthe group consisting of crosslinked reaction products of a crosslinkingagent with the following polymers: a polyvinyl alcohol; a polyvinylalcohol and a vinyl polymer; a cellulosic polymer; a polyurethane; anepoxy polymer; an acrylic polymer; and a vinyl polymer.

In one embodiment of the methods of the present invention, theinfrared-absorbing layer further comprises a catalyst in addition to oneor more polymers and an infrared-absorbing sensitizer.

In one embodiment of the methods of this invention, theinfrared-absorbing layer comprises one or more polymers, aninfrared-absorbing sensitizer, and an organic sulfonic acid component,preferably a component of an amine-blocked p-toluenesulfonic acid. Theterm “organic sulfonic acid,” as used herein, refers to organiccompounds that have at least one sulfonic acid moiety, —SO₃H, covalentlybonded to a carbon atom of the organic compound. The term “organicsulfonic acid component,” as used herein, pertains to free organicsulfonic acids and also pertains to the free organic sulfonic acidsformed when a blocked or latent organic sulfonic acid catalyst, isdecomposed, such as by heat or by radiation, to form a free or unblockedorganic sulfonic acid to catalyze the desired curing reaction, as iswell known in the art. The weight of the free organic sulfonic acid thatmay be obtained from the blocked or latent organic sulfonic acidcatalyst is used herein to calculate the weight per cent of the organicsulfonic acid component based on the total weight of polymers present inthe coating layer. As is well known in the art, the blocked organicsulfonic acid catalysts may be an adduct or complex of an organicsulfonic acid with a complexing material, such as an amine, and themolar ratios of the organic sulfonic acid and the complexing materialmay vary widely, such as, for example, from 1.0:0.5 to 1.0:2.0.Alternatively, the blocked organic sulfonic acid catalysts may be areaction product of an organic sulfonic acid with a suitable material,such as, for example, with an alcohol to provide the blocked catalyst inthe form of an ester of an organic sulfonic acid. A wide variety ofblocked or latent organic sulfonic acid catalysts are known and may beutilized in the present invention to provide the organic sulfonic acidcomponent. Examples of suitable blocked or latent organic sulfonic acidcatalysts that provide suitable organic sulfonic acid componentsinclude, but are not limited to, amine-blocked organic sulfonic acidssuch as, for example, described in U.S. Pat. Nos. 4,075,176; 4,200,729;4,632,964; 4,728,545; 4,812,506; 5,093,425; 5,187,019; 5,681,890; and5,691,002; esters of an organic sulfonic acid as, for example, describedin U.S. Pat. Nos. 4,192,826; 4,323,660; 4,331,582; 4,618,564; 5,102,961;5,364,734; and 5,716,756; reaction products of an organic sulfonic acidand a glycidamide as, for example, described in U.S. Pat. No. 4,839,427;and amides of an organic sulfonic acid as, for example, described inU.S. Pat. No. 4,618,526. Instead of the free or unblocked organicsulfonic acid in the coating solutions to be applied to a substrate, theblocked or latent organic sulfonic acid catalysts are typically utilizedto crosslink coatings in order to provide a stable shelf life to thecoating solution by reducing the viscosity buildup due to prematurecrosslinking and because of the better coating uniformity and waterresistance often obtained in the finished coating layers.

An example of an amine-blocked p-toluenesulfonic acid (PTSA) is NACURE2530, a tradename for a catalyst available from King Industries,Norwalk, Conn. These PTSA-based catalysts assist in the curing of, forexample, CYMEL 303, a trademark for melamine crosslinking agentsavailable from Cytec Industries, Inc., Wayne, N.J., with polymers in theinfrared-absorbing layer and, when there is penetration of the catalystinto underlying layers such as the hydrophilic layer, with polymers inthese underlying layers. Examples of these crosslinkable polymers areAIRVOL 125, a trademark for polyvinyl alcohol polymers available fromAir Products, Allentown, Pa., and UCAR WBV-110, a trademark for a vinylcopolymer water-based dispersion available from Union CarbideCorporation, Danbury, Conn. To calculate the weight per cent of organicsulfonic acid component in the infrared-absorbing layer of the presentinvention, the weight of organic sulfonic acid component(p-toluenesulfonic acid constitutes 25 per cent by weight of NACURE 2530in the examples of the present invention) is divided by the total dryweight of polymers present based on the combined weight of crosslinkingagent and any polymers present.

A wide variety of organic sulfonic acid components are known and may beutilized in the present invention. Examples of suitable organic sulfonicacid components include, but are not limited to, organic sulfonic acidshaving a pKa below 4, such as, for example, p-toluenesulfonic acid,dodecylbenzenesulfonic acid, dinonylnaphthalene sulfonic acid,tridecylbenzene sulfonic acid, methane sulfonic acid, polystyrenesulfonic acid, and dodecylbenzenedisulfonic acid. In one embodiment, theorganic sulfonic acid component of the present invention is an aromaticsulfonic acid. In a preferred embodiment, the organic sulfonic acidcomponent is p-toluenesulfonic acid (PTSA). In one embodiment, theorganic sulfonic acid component of the present invention is a componentof a blocked or latent organic sulfonic acid catalyst, preferably anamine-blocked organic sulfonic acid. The term “amine,” as used hereinpertains to ammonia, as well as to aliphatic primary, secondary, andtertiary amines, including heterocyclic amines having a saturated ring.

Surprisingly, it has been found that significantly increased levels ofan organic sulfonic acid component, such as the p-toluenesulfonic acidin NACURE 2530 to weight per cents greater than 12% and preferably than25%, of the total weight of polymers present in conjunction with largeamounts of water-dispersible infrared-absorbing, carbon black sensitizerin the infrared-absorbing layer to weight per cents of sensitizergreater than 55% of the dry layer weight provide significantimprovements in the ease of cleaning the laser-exposed areas, in asignificant reduction of the laser power required for imaging while notaffecting durability and adhesion of the ink-accepting areas of theplate during long press runs, and in the fine image resolution andprinting quality that can be achieved. Even more surprisingly, the largereduction in laser imaging power results in lower imaging temperaturesand most importantly, in the absence of ablation of the infraredabsorbing layer. The absence of ablation during imaging results in asignificant reduction, if not elimination, of noxious vapors andairborne debris, thus eliminating the need for expensive vapor anddebris removal and capture equipment on the laser imaging system. Thesebenefits from high levels of organic sulfonic acid components may beobtained without any significant disadvantages, such as loss in imageresistance to solubilization or removal by water, by the fountainsolution, or by a cleaning solution.

Thus, in a preferred embodiment, the infrared-absorbing layer comprisesgreater than 25 weight per cent of an organic sulfonic acid component.In one embodiment, the organic sulfonic acid component is present in anamount of 25 to 75 weight per cent of the total weight of polymerspresent in the infrared-absorbing layer of the printing member of themethods of the present invention. In another embodiment, the organicsulfonic acid component is present in an amount of 35 to 55 weight percent of the total weight of polymers present in the infrared-absorbinglayer. In one embodiment, the infrared-absorbing layer comprises greaterthan 5% by weight of organic sulfonic acid component. In one embodiment,the infrared-absorbing layer comprises greater than 12% by weight of theorganic sulfonic acid component.

Substrates for Methods of Preparing and Imaging Lithographic PrintingMembers

Suitable substrates for the support substrate of the lithographicprinting members of the methods of this invention may be a variety ofdifferent materials, including those known in the art as substrates forlithographic printing plates, such as, for example, metals, papers, andpolymeric films. Since the hydrophilic layer 104 as illustrated in FIGS.2, 3A, and 3B, is not soluble in water, in a cleaning solution, or inthe fountain solution, and further is not removed during the laserimaging and cleaning process, the substrate 106 does not need to behydrophilic to provide the discrimination between the ink-accepting ornon-hydrophilic image areas of the surface layer and the water-acceptingor hydrophilic background areas of the plate needed for wet lithographicprinting. The term, “hydrophilic,” as used herein, pertains to theproperty of a material or a composition of materials that allows it topreferentially retain water or a water-based fountain solution in wetlithographic printing while the non-hydrophilic, ink-accepting materialsor composition of materials on the surface of the plate preferentiallyretain the oily material or ink. Thus, the substrate 106 either may behydrophilic or may be non-hydrophilic/ ink-accepting when a hydrophiliclayer such as hydrophilic layer 104 is interposed between theinfrared-absorbing layer and the substrate. For example, the substratemay be a metal substrate, such as aluminum, comprising a non-hydrophilicpolymer layer on at least one surface of the substrate to provide anon-hydrophilic metal substrate.

Suitable metals include, but are not limited to, aluminum, copper,steel, and chromium, preferably that have been rendered hydrophilicthrough graining or other treatments. The printing members of thisinvention preferably use an anodized aluminum support substrate.Examples of such supports include, but are not limited to, aluminumwhich has been anodized without prior graining, aluminum which has beengrained and anodized, and aluminum which has been grained, anodized, andtreated with an agent effective to render the substrate hydrophilic, forexample, treatment to form a silicate layer. It is preferred in thisinvention to use aluminum, which has been grained, anodized, and treatedwith a hydrophilic material. For increased ease of cleanability andbetter image resolution, in a preferred embodiment, the metal substrateis an aluminum substrate comprising a surface of uniform,non-directional roughness and microscopic depressions, which surface isin contact to the hydrophilic layer and, more preferably, this surfaceof the aluminum substrate has a peak count in the range of 300 to 450peaks per linear inch which extend above and below a total bandwidth of20 microinches, as described for example, in International PublicationNo. WO 97/31783. In one embodiment, the aluminum substrate is SATINFINISH aluminum litho sheet, a trademark for aluminum sheets availablefrom Alcoa, Inc., Pittsburgh, Pa.

A wide variety of papers may be utilized. Typically, these papers havebeen treated or saturated with a polymeric treatment to improvedimensional stability, water resistance, and strength during the wetlithographic printing. Examples of suitable polymeric films include, butare not limited to, polyesters such as polyethylene terephthalate andpolyethylene naphthalate, polycarbonates, polystyrene, polysulfones, andcellulose acetate. A preferred polymeric film is polyethyleneterphthalate film, such as, for example, the polyester films availableunder the trademarks of MYLAR and MELINEX polyester films from E. I.Dupont de Nemours Co., Wilmington, Del. It is preferred that thepolymeric film substrate be wettable by aqueous coating fluids and beadherable to hydrophilic polymers. A number of commercial suppliers ofpolymeric films provide products with these properties. Preferredthicknesses for support substrate 106 range from 0.003 to 0.02 inches,with thicknesses in the range of 0.005 to 0.015 inches beingparticularly preferred.

Methods of Preparing and Imaging Lithographic Printing Members Having aTwo Layer Product Configuration with a Primer Layer

The present invention utilizes novel infrared-absorbing and hydrophiliclayers with special attention to the interface between these two layersto enhance the laser imaging sensitivity, absence of ablation andairborne byproducts, printing quality, cleanability, press durability,ink-accepting image adhesion, and fine dot resolution of lithographicprinting plates. Referring to FIG. 2, another aspect of this inventionis the incorporation of a primer layer interposed between theinfrared-absorbing layer 102 and the hydrophilic layer 104, wherein theprimer layer comprises an adhesion-promoting agent. Suitableadhesion-promoting agents include, but are not limited to, organicsulfonic acid components, zirconium compounds, titanates, and silanes.In one embodiment, the organic sulfonic acid component of theadhesion-promoting agent in the primer layer is an aromatic sulfonicacid. In a preferred embodiment, the organic sulfonic acid component ofthe adhesion-promoting agent in the primer layer is p-toluenesulfonicacid.

In one embodiment, the organic sulfonic acid component in the primerlayer interposed between the infrared-absorbing layer 102 and thehydrophilic layer 104 is present in an amount of 2 to 100 weight percent of the primer layer, preferably in an amount of 50 to 100 weightper cent of the primer layer, and most preferably in an amount of 80 to100 weight per cent of the primer layer.

In one embodiment, the thickness of the primer layer interposed betweenthe ablative-absorbing layer 102 and the hydrophilic layer 104 is fromabout 0.01 to about 2 microns, and preferably from about 0.01 to about0.1 microns. When this primer layer comprising an organic sulfonic acidcomponent is present, increased levels of an organic sulfonic acidcomponent in the infrared-absorbing layer 102 may not be necessary toprovide the multiple benefits desired, and the level of an organicsulfonic acid component in the infrared-absorbing layer 102 may be lessthan 25 weight per cent of the total weight of the polymers present inthe infrared-absorbing layer or may even be negligible.

In one embodiment, the adhesion-promoting agent of the primer layer isammonium zirconyl carbonate such as, for example, BACOTE 20. BACOTE 20is a zirconia sol from Magnesium Elektron, Inc., with a weightequivalent of 20% zirconium oxide. The cured residue of an appliedBACOTE 20 solution is reported to be water-insoluble and to haveexcellent adhesion to chrome substrates and photopolymer coatings inphotopolymer coated lithographic printing plates and may also havehydrophilic properties depending on the overlying coating, as describedin U.S. Pat. Nos. 4,522,912 and 4,581,825. In another embodiment, theadhesion-promoting agent of the primer layer is zirconium propionate.Other suitable zirconium compounds in the primer layer of the presentinvention include, but are not limited to, those zirconium-basedadhesion promoters described in “The Use of Zirconium in SurfaceCoatings,” Application Information, Sheet 117 (Provisional), by P. J.Moles, Magnesium Elektron, Inc., Flemington, N.J.

Thus, one aspect of the methods of imaging a wet lithographic printingmember of this invention pertains to the inclusion of a primer layerinterposed between the hydrophilic layer and the infrared-absorbingsurface layer to further enhance the desirable properties of theinfrared-absorbing and hydrophilic layers and their interface whichgreatly influence the amount of airborne materials produced during laserimaging, the speed of laser imaging, the ease of cleanability by waterduring removal of the laser-exposed areas, and the image resolution anddurability, as described herein. In one embodiment, the methods ofimaging a wet lithographic printing member of this invention comprisethe steps of (a) providing a positive working lithographic printingmember, which positive working member comprises a substrate, ahydrophilic layer overlying the substrate, an ink-accepting andinfrared-absorbing surface layer overlying the hydrophilic layer, and aprimer layer interposed between the hydrophilic layer and the surfacelayer; wherein the surface layer is characterized by absorption ofinfrared imaging radiation, by being not removable by cleaning withwater or a cleaning solution prior to absorption of infrared imagingradiation, and by being adapted to form a wet lithographic printingsurface as a result of an imagewise exposure to absorbable infraredradiation and subsequent removal of the exposed areas of the surfacelayer by cleaning with water or the cleaning solution to reveal theunderlying hydrophilic layer; the primer layer comprising anadhesion-promoting agent; and the hydrophilic layer is characterized bybeing not removable by cleaning with water or the cleaning solution; (b)exposing the positive working member of step (a) to absorbable infraredradiation using an infrared-emitting laser to effect absorption ofinfrared radiation in the laser-exposed areas of the surface layer thatis sufficient to cause the surface layer in the laser-exposed areas tobecome removable by cleaning with water or the cleaning solution butinsufficient to remove by ablation greater than 10% by weight of thesurface layer in the laser-exposed areas; and (c) removing, with wateror the cleaning solution, the laser-exposed areas of the surface layerto reveal the underlying hydrophilic layer. In one embodiment, thethickness of the primer layer is from about 0.01 to about 0.1 microns.In one embodiment, the adhesion-promoting agent comprises a crosslinked,polymeric reaction product of a hydrophilic polymer and a crosslinkingagent. In one embodiment, the primer layer further comprises a catalyst.In one embodiment of the methods, the hydrophilic layer is characterizedby the absence of removal of the hydrophilic layer in the laser-exposedareas during steps (b) and (c). In a preferred embodiment, theabsorption of infrared radiation in the laser-exposed areas of thesurface layer of step (b) is sufficient to cause the surface layer inthe laser-exposed areas to become removable by cleaning with water orthe cleaning solution but insufficient to remove by ablation greaterthan 5% by weight, more preferably greater than 2% by weight, and mostpreferably none, of the infrared-absorbing or surface layer in thelaser-exposed areas.

Methods of Preparing and Imaging Lithographic Printing Members Having aThree Layer Product Configuration

A three layer configuration for the lithographic printing members foruse in the methods of preparing wet lithographic member of the presentinvention is illustrated in FIG. 4. This three layer productconfiguration comprises an ink-accepting surface layer 100, aninfrared-absorbing layer 102, and a hydrophilic layer 104 on a substrate106. FIGS. 5A and 5B show one embodiment of the methods of preparing wetlithographic printing members of this invention for the lithographicprinting member of FIG. 4. As shown in FIG. 5A, laser imaging radiationinteracts with layers 100, 102, and 104 without generating ablationby-products, such as airborne debris and vapors, and leaves alaser-exposed area 108 on the hydrophilic layer 104. As shown in FIG.5B, the laser-imaged plate is then cleaned with a cleaning solution,such as water, in order to remove laser exposed area 108, therebyexposing the surface 110 of the hydrophilic layer 104. None of thishydrophilic layer 104 is ablated during the laser imaging steps, asillustrated in FIG. 5A. Also, none of this hydrophilic layer 104 isremoved during the cleaning step, as illustrated in FIG. 5B. Theablation-resistant and cleaning-resistant properties of this hydrophiliclayer, and the interfacial properties between the hydrophilic layer andthe overlying infrared-absorbing layer, are among the key features ofthis invention, as described herein.

Thus, one aspect of the methods of imaging a wet lithographic printingmember of the present invention pertains to the inclusion of anadditional ink-accepting surface layer overlying the infrared-absorbinglayer to provide a basic three layer product configuration ofink-accepting surface layer/infrared-absorbing layer/hydrophilic layeron the substrate. This additional ink-accepting surface layer may beuseful in achieving the best overall balance of properties, such as thespeed of laser imaging, the ease of cleanability by water during removalof the laser-exposed areas, and particularly the image resolution anddurability. In one embodiment, the method of imaging a wet lithographicprinting member comprises the steps of (a) providing a positive workinglithographic printing member, which positive working member comprises asubstrate, a hydrophilic layer overlying the substrate, aninfrared-absorbing layer overlying the hydrophilic layer, and anink-accepting surface layer overlying the infrared-absorbing layer; thesurface layer being characterized by the absence of ablation fromabsorption of infrared imaging radiation; the infrared-absorbing layerbeing characterized by absorption of imaging radiation; the surfacelayer and the infrared-absorbing layer being characterized by being notremovable by cleaning with water or a cleaning solution prior to theabsorption of infrared imaging radiation and by being adapted to form awet lithographic printing surface as a result of an imagewise exposureto absorbable infrared radiation and subsequent removal of the exposedareas of the surface layer and the infrared-absorbing layer by cleaningwith water or the cleaning solution to reveal the underlying hydrophiliclayer; and the hydrophilic layer being characterized by being notremovable by cleaning with water or the cleaning solution; (b) exposingthe positive working member of step (a) to absorbable infrared radiationusing an infrared-emitting laser to effect absorption of infraredradiation in the laser-exposed areas of the infrared-absorbing layerthat is sufficient to cause the surface layer and the infrared-absorbinglayer in the laser-exposed areas to become removable by cleaning withwater or the cleaning solution but insufficient to remove by ablationgreater than 10% by weight of the combined surface layer andinfrared-absorbing layer in the laser-exposed areas; and (c) removing,with water or the cleaning solution, the laser-exposed areas of thesurface layer and the infrared-absorbing layer to reveal the underlyinghydrophilic layer. In one embodiment of the methods, the hydrophiliclayer is characterized by the absence of removal of the hydrophiliclayer in the laser-exposed areas during steps (b) and (c). In apreferred embodiment, the absorption of infrared radiation in thelaser-exposed areas of the infrared-absorbing layer of step (b) issufficient to cause the ink-accepting surface layer and theinfrared-absorbing layer in the laser-exposed areas to become removableby cleaning with water or the cleaning solution but insufficient toremove by ablation greater than 5% by weight, more preferably greaterthan 2% by weight, and most preferably none, of the combinedink-accepting surface layer and infrared-absorbing layer in thelaser-exposed areas.

Suitable infrared-absorbing layers, hydrophilic layers, and substratesfor the three layer product configuration, as illustrated in oneembodiment in FIGS. 4, 5A, and 5B, are as described herein for thecorresponding layers in the two layer product configuration of thelithographic printing members of the methods of the present invention.For example, in one embodiment, the infrared-absorbing layer of thethree layer design is ink-accepting. Since the surface layer of thethree layer product configuration is ink-accepting, in anotherembodiment, the infrared-absorbing layer of the three layer design ischaracterized by not accepting ink and by accepting water on a wetlithographic printing press. Also, the three layer product configurationmay optionally have a primer layer interposed between theinfrared-absorbing layer and the hydrophilic layer as described hereinfor the corresponding primer layer on the two layer productconfiguration.

Ink Accepting Surface Layer for Methods of Preparing and ImagingLithographic Printing Members Having a Three Layer Product Configuration

The primary characteristics of the ink-accepting surface layer 100, asillustrated in FIGS. 4, 5A, and 5B, are its oleophilicity andhydrophobicity, ability to be imaged in conjunction with the underlyinginfrared-absorbing layer 102, resistance to solubilization or removal bywater, cleaning solutions, and fountain solutions, and durability on theprinting press. For example, the ink-accepting surface layer should beresistant to the press chemistry, which may include organic solvents.Suitable polymers utilized in this layer should have excellent adhesionto the infrared-absorbing layer 102 and high wear resistance. They canbe either water-based or solvent-based polymers. Ink-accepting surfacelayer 100 does not need to comprise an infrared-absorbing sensitizer andis characterized by the absence of ablative absorption by the infraredimaging radiation. This layer also may include a crosslinking agent,which provides improved bonding to the infrared-absorbing layer 102 andincreased durability of the plate for extremely long print runs.

Suitable polymers include, but are not limited to, cellulosics such asnitrocellulose, acrylic polymers, polyurethanes, and epoxy polymers. Forexample, polyurethane based materials are typically extremely tough andmay have thermosetting or self-curing capability. Mixing and coatingapplication methods known in the art may be used to prepare theink-accepting surface layer. In one example of a coating for theink-accepting surface layer, a mixture of nitrocellulose polymer andhexamethoxymethylmelamine crosslinking agent in a suitable solvent blendis combined, followed by the addition of a suitable amine-blockedp-toluenesulfonic acid catalyst to form the finished coating mix. Thecoating mix is then applied to the infrared-absorbing layer 102 usingone of the conventional methods of coating application, such as wirewound rod coating, reverse roll coating, gravure coating, squeezecoating, and slot die coating, and subsequently dried to remove thevolatile liquids and cured to form a coating layer.

Polymeric systems containing components in addition to a cellulosic orother primary polymer may also be combined to form the ink-acceptingsurface layer 100. For example, an epoxy polymer may be added to anitrocellulose polymer in the presence of a crosslinking agent and acatalyst. After coating, the layer is dried and preferably cured at atemperature of between 75° C. and 175° C.

In one embodiment of the methods of imaging a wet lithographic printingmember with a three-layer product design of this invention, theink-accepting surface layer comprises a crosslinked, polymeric reactionproduct of a polymer and a crosslinking agent. Suitable polymers for thecrosslinked, polymeric reaction product include, but are not limited to,cellulosics, acrylic polymers, polyurethanes, and epoxy polymers. In oneembodiment, the ink-accepting surface layer further comprises an organicsulfonic acid component. In one embodiment, the weight of theink-accepting surface layer is from about 0.05 to about 0.5 g/m². In apreferred embodiment, the weight of the ink-accepting surface layer isfrom about 0.1 to about 0.3 g/m².

Imaging Apparatus

Imaging apparatus suitable for use in conjunction with the presentinvention include, but are not limited to, known laser imaging devicessuch as infrared laser devices that emit in the infrared spectrum.Examples include the PEARLSETTER 74, a trademark for laser imagingequipment available from PRESSTEK, Inc., Hudson, N.H., and the CREOTRENDSETTER 1.7X, a trademark for laser imaging equipment available fromCreo, Inc., Burnaby, British Columbia. Laser outputs can be provideddirectly to the plate surface via lenses or other beam-guidingcomponents, or transmitted to the surface of a printing plate from aremotely sited laser using a fiber-optic cable. The imaging apparatuscan operate on its own, functioning solely as a platemaker, or it can beincorporated directly into a lithographic printing press. In the lattercase, a cleaning step is necessary prior to or during the startup of theprinting operation.

The laser-induced imaging of the wet lithographic printing plates of thepresent invention may be carried out using a wide variety of laserimaging systems known in the art of thermal laser-induced imaging,including, but not limited to, the use of continuous and pulsed lasersources of infrared wavelengths. Preferably, the laser-induced imagingof this invention is carried out utilizing a continuous laser source ofnear-infrared radiation, such as, for example, with a diode laseremitting at 830 nm.

Imaging Techniques

A lithographic printing plate of the methods of the present invention isselectively exposed, in a pattern representing an image, to the outputof an imaging laser, which is scanned over the plate. Referring to FIG.5A, radiative laser output exposes the infrared-absorbing layer 102 andthe ink-accepting surface layer 100 in a desired imagewise pattern. Asshown in one embodiment in FIG. 5A, imaging radiation transforms layers100 and 102 without generating ablation products such as ablation debrisand airborne particles, leaving a laser exposed, removable area 108 onthe hydrophilic layer 104. The laser-exposed plate is then cleaned withwater or a cleaning solution in order to remove laser exposed area 108,thereby exposing the surface of the hydrophilic layer 104 in area 110 asshown in FIG. 5B.

Similarly, for a two layer product configuration, referring to FIG. 3A,infrared imaging radiation exposes the infrared-absorbing layer 102 in adesired imagewise pattern. As shown in one embodiment in FIG. 3A,infrared imaging radiation transforms layer 102 without generatingablation debris and airborne particles, leaving a laser exposed,removable area 108 on the hydrophilic layer 104. The laser-exposed plateis then cleaned with water or a cleaning solution in order to removelaser-exposed area 108, thereby exposing the surface of the hydrophiliclayer 104 in area 110, as shown in FIG. 5B.

One aspect of the present invention pertains to a method of imaging awet positive working lithographic printing member having aninfrared-absorbing imaging layer, which member can be imaged withoutusing sufficient energy to form a lithographic printing surface by laserablation removal of the imaging layer and without the use of alkaline orsolvent developing solutions; the method comprising the steps of (a)providing a positive working lithographic printing member, the positiveworking member comprising a substrate, a hydrophilic layer overlying thesubstrate, and an ink-accepting, infrared radiation-absorbing,heat-sensitive, polymeric surface layer which is not readily removablefrom the hydrophilic layer by cleaning with water or a cleaning solutionprior to the absorption of infrared imaging radiation and is adapted toform a wet lithographic printing surface as a result of an imagewiseexposure to absorbable infrared radiation and subsequent easy removal ofthe exposed areas of the surface layer by cleaning with water or thecleaning solution to reveal the underlying hydrophilic layer; and thehydrophilic layer being characterized by being not removable by cleaningwith water or the cleaning solution; (b) exposing the positive workingmember to absorbable infrared radiation using an infrared emitting laserto effect absorption of infrared radiation and localized generation ofheat in the laser-exposed areas of the surface layer that is sufficientto cause the surface layer in the laser-exposed areas to becomeremovable by cleaning with water or the cleaning solution butinsufficient to remove by ablation greater than 10% by weight,preferably greater than 5%, more preferably greater than 2%, and mostpreferably none, of the surface layer in the laser-exposed areas; and(c) removing, with water or the cleaning solution, the laser-exposedareas of the surface layer to reveal the underlying hydrophilic layer.

Another aspect of the present invention pertains to wet lithographicprinting members prepared according to the methods of this invention, asdescribed herein and as illustrated, for example, in FIGS. 2, 3A, 3B, 4,5A, and 5B.

Thus, for example, one aspect of the present invention pertains to apositive working lithographic printing member which comprises (a) asubstrate; (b) a hydrophilic layer overlying the substrate; and (c) anink-accepting surface layer overlying the hydrophilic layer; wherein thesurface layer is characterized by absorption of infrared imagingradiation, by being not removable by cleaning with water prior to theabsorption of infrared imaging radiation, and by being adapted to form awet lithographic printing surface as a result of an imagewise exposureto absorbable infrared radiation that removes by ablation less than 10%by weight, preferably less than 5%, more preferably less than 2%, andmost preferably none, of the surface layer in the laser-exposed areasand of subsequent removal of the laser-exposed areas of the surfacelayer by cleaning with water to reveal the underlying hydrophilic layer;and said hydrophilic layer is characterized by being not removable bycleaning with water.

EXAMPLES

Several embodiments of the present invention are described in thefollowing examples, which are offered by way of description and not byway of limitation.

Example 1

A lithographic printing plate substrate with a hydrophilic thermalbarrier layer in accordance with the invention, as illustrated by layer104 in FIG. 2 of this application, was prepared on a production line bycoating a web of grained, anodized, silicated aluminum. The hydrophilicthermal barrier layer had the composition on a dry weight basis shownbelow.

Hydrophilic Layer Composition AIRVOL 325 4.28 BACOTE 20 1.75 GLYCEROL0.17 TRITON X-100 0.07 BYK 333 0.10

AIRVOL 325 is a highly hydrolyzed grade of polyvinyl alcohol from AirProducts & Chemical, Inc. BACOTE 20 is an alkaline aqueous solution ofstabilized ammonium zirconium carbonate containing 20% ZrO₂ fromMagnesium Elektron, Inc. of Flemington, N.J. TRITON X-100 is ahydrocarbon surfactant from Rohm & Haas. BYK 333 is a trademark for asilicone surfactant from Byk Chemie USA of Wallingford, Conn.

The above components were mixed with water to make a 6.3% by weightsolution. This solution was applied to the aluminum web with a # 18 wirewound rod and dried for 60 seconds at 285° F. with hot air and thenpartially cured using infrared heat for 60 seconds at a temperatureadequate to raise the temperature of the web to 310 to 320° F. Thedegree of cure of the hydrophilic layer was tested by rubbing very hardwith a water wet WEBRIL wipe, a trademark for a lint-free clothavailable from Veratec Corporation, Walpole, Mass. The wet rubs eachinvolve a durable pass back and forth across the coating layer so that30 durable rubs in the wet rub resistance tests of this inventionactually involve a total of 60 passes or wet rubs across the coatinglayer. Optimum performance characteristics of the final imaged plate isachieved in some embodiments when no more than 30 double rubs,preferably 10 to 15 double rubs, are needed to remove the hydrophiliclayer down to the substrate when tested after coating, drying, andpartially curing the hydrophilic layer but prior to subsequent coating,drying, and curing steps.

Example 2

Two layer wet lithographic printing plates, as illustrated in FIG. 2,were prepared by applying ink-accepting, infrared-absorbing layers ofthe following compositions to the hydrophilic thermal barrier coatedaluminum substrate prepared as described in Example 1. Example 2A is acomparative example that represents a wet lithographic printing platethat only images acceptably by an ablation-imaging process. Example 2Bthrough 2F are examples of lithographic printing plates in accordancewith the present invention.

Composition of Dry Coating (wt %) 2A 2B 2C 2D 2E 2F AIRVOL A325 25.314.3 12.7 14.0 6.9 6.2 CYMEL 303 4.3 0.6 0.6 2.0 1.0 0.9 WITCOBOND 2400.0 0.0 12.7 4.1 2.0 1.8 JONCRYL 540 0.0 0.0 0.0 0.0 50.6 45.5 Bindersubtotal 29.6 14.9 26.0 20.1 60.5 54.4 BONJET CW-1 53.1 69.6 60.2 69.034.0 40.7 TRITON X100 2.0 6.7 6.0 2.4 1.2 1.1 NACURE 2530 15.4 8.7 7.88.6 4.2 3.8 TOTAL 100.0 99.9 100.0 100.1 99.9 100

CYMEL 303 is a hexamethoxymethylmelamine crosslinking agent supplied byCytec Industries, Inc. BONJET CW-1 is a tradename for aself-crosslinking polyurethane dispersion of approximately 100 nm carbonparticles that have a high active hydrogen content that is supplied byOrient Chemical. NACURE 2530 is an amine-blocked p-toluenesulfonic acidcatalyst in an isopropand /methanol blend supplied by King Industries,Inc. JONCRYL 540 is a trademark for acrylic latex polymers supplied byS. C. Johnson, Racine, Wis. JONCRYL 540 has an acid number of 49 and ahydroxyl number 42. The above six compositions are on a dry weightbasis. These compositions were coated at the % solids described belowafter adding water and 2-butoxythenol to provide an overall solventsystem of about 95% by weight of water and 5% by weight of a solventblend of 2-butoxyethanol, isopropanol, and methanol. Dissolution of thepolymers in the solvent system, mixing of the ingredients, and coatingapplication were done utilizing methods known in the art for these typesof materials and coating formulations.

The infrared-absorbing layer of Example 2A was applied to thehydrophilic barrier coated aluminum substrate as a 4% solids fluid usinga #4 Meyer wire wound rod on a production coating machine and dried at145° C. for two minutes. When exposed on a Creo TRENDSETTER 1.7X laserimaging unit at 100 RPM and 14 watts (400 mj/cm² exposure energy),significant ablation occurred as evidenced by the reduced density in thelaser-exposed areas, the high level of odor emanating from the surface,and the easily detected loose debris seen when wiped with a dry finger.At this laser exposure level, the plate did not completely clean up whenscrubbed by water using a series of molleton rolls. By increasing thelaser exposure level by exposing the plate at 65 RPM and 14 watts (610mj/cm² exposure energy), the plate of Example 2A did completely cleanup, but there was increased ablation as particularly evidenced by theincreased level of loose debris that could be wiped off with a dryfinger.

The infrared-absorbing layer of Examples 2B, 2C, and 2D had lesspolyvinyl alcohol binder and more water-dispersible black than theinfrared-absorbing layer of Example 2A. Examples 2B, 2C, and 2D werecoated at 1.6, 2.0 and 2.5% solids, respectively. All three of theseinfrared-absorbing coatings were applied to the hydrophilic layerutilizing a #4 Meyer wire wound rod and dried at 145° C. for twominutes. These all cleaned up well with water when imaged on the CREOlaser-imaging unit at various faster imaging speeds than that used withExample 2A. The imaging speeds used were 40 to 100% faster than usedwith Example 2A. In contrast to Example 2A which showed significantablation of at least 40% of the infrared-absorbing layer when imaged at100 RPM and 14 watts, Examples 2B, 2C, and 2D over the range of coatingweights evaluated showed less than 10% ablation of theinfrared-absorbing layer when imaged over the 140 to 200 RPM and 14watts range (200 to 280 mj/cm² exposure energy), and still exhibitedexcellent cleanability with water for completely removing thelaser-exposed areas to provide a wet lithographic plate with excellentimage resolution and fair durability.

The durability of Example 2D was improved from fair to good by dryingand curing the infrared-absorbing layer at 175° C. for two minutesinstead of at 145° C. for two minutes. Similar to observations inExample 1, the drying and curing conditions for the infrared-absorbingcoating layer may have a significant effect on the overall quality ofthe lithographic printing member, particularly its ease of cleaning,image resolution, and durability on the press. Generally, these dryingand curing conditions and the degree of curing of each coating layer ateach stage of the process of preparing the lithographic printing memberare determined by experimentation and chosen so as to optimize theoverall quality. Also, after laser exposure and cleaning to remove thelaser-exposed areas, the printing member may be further heated toincrease the durability on the press. The durability on the press wasestimated by wet rub resistance testing. Wet rub resistance is evaluatedby measuring the finest lines or dots on the plate that survive 50 wetrubs with a WEBRIL cloth, a trademark for a lint-free cloth availablefrom Veratec Corporation, Walpole, Mass., which has been wet with water.The wet rubs each involve a double pass back and forth across the imagedareas so that 50 wet rubs in the wet rub resistance tests of thisinvention actually involve a total of 100 passes or wet rubs across theimaged area. The wet rub resistance testing is also done with a WEBRILcloth which has been wet with methyl ethyl ketone.

In the resolution and wet rub resistance testing of this invention, theimage areas are of two types: (1) narrow lines in the form of a seriesof pixels with the width of the lines based on the number of pixelscomprising the width, and (2) half tone dots at 150 lines per inch (lpi)halftone screen imaging. Approximate sizes of these image areas are asfollows. One pixel lines are 15 microns wide, and 3 pixel lines are 40microns wide. 2% Dots are 15 microns in diameter, 3% dots are 20 micronsin diameter, 4% dots are 25 microns in diameter, 5% dots are 35 micronsin diameter, and 10% dots are 60 microns in diameter. The smaller thewidths of the pixel lines and the smaller the diameters of the dot sizesthat can be achieved and maintained on the plate are the better forprinting quality and press run length with acceptable quality. Thus,achieving a 1 pixel wide line image after cleaning and maintaining the 1pixel wide line image through the wet rub resistance test is the bestresult for printing quality. Similarly, achieving a 2% dot image or adot that is about 15 microns in diameter after cleaning and maintainingthe 2% dot image through the wet rub resistance test is an excellentresult for printing quality, and much more desirable compared tomaintaining only 5% or 10% dots as the best dot images.

The image resolution was measured as the finest lines or dots of imageor ink-acceptable areas that are achieved on the plate after the laserimaging exposure and subsequent cleaning steps.

The weight loss in the metal plate from laser ablation imaging isdifficult to measure quantitatively, but may be estimated by comparisonto calibrated standards from the same coatings on a plastic substrate,such as polyethylene terephthalate film, measured on a high precisionanalytical balance before and after laser exposure.

The infrared-absorbing layer of Examples 2E and 2F were coated over arange of lower and similar coating weights compared to theinfrared-absorbing layer of Example 2A. Example 2E was coated at 4.8%solids, and Example 2F was coated at both 2.6% and 5.2% solids. All ofthese coatings were applied using a #4 Meyer wire wound rod. In order toincrease the image resolution and durability of the imaged plate,coatings 2E and 2F were dried and cured at 175° C. for two minutes. Inspite of the increased amount of binder present and the lower amount ofwater-dispersible carbon black present, Examples 2E and 2F cleaned upwell with water after imaging on the Creo laser imaging unit over arange of 120 to 160 RPM and 14 watts (250 to 330 mj/cm² exposureenergy). Before cleaning with water, these plates showed less than 10%ablation of the infrared-absorbing layer and, after cleaning, showedexcellent wet lithographic plate properties, particularly excellentimage resolution and excellent durability.

Substituting JONCRYL 138, which has no hydroxyl content and an acidnumber of 60, for the JONCRYL 540 in Example 2F at 5.2% solids, resultedin similar but not quite as good performance as found with Example 2F.

These results show the flexibility possible for the infrared-absorbinglayer formulations and processing conditions of the non-ablative, extrahigh imaging speed, and easily cleaned wet lithographic printing membersand methods of the present invention.

When imaged on the Creo laser imaging unit at the laser exposureconditions described above, in addition to the visible ablative weightloss of the infrared-absorbing layer, Example 2A, the comparativelaser-ablative example, had considerable weakly bound debris in thelaser-exposed areas which could be removed by a dry rub with a finger ora cloth and also had a strong odor when evaluated after removing it fromthe laser imaging unit right after the laser exposure. In contrast,Examples 2B to 2F had little or no weakly bound debris that could beremoved by a dry rub with a finger or a cloth and also had no strongodor at any time during or after the laser exposure when imaged asdescribed above.

If the laser exposure dwell time on Examples 2B to 2F is increasedsignificantly on the Creo laser imaging unit, such as to an exposure at65 RPM to 85 RPM and 14 watts (470 to 610 mj/cm² exposure energy), theywill show ablation of more than 20% of the infrared-absorbing layertogether with the development of some odor and a moderate amount ofweakly bound debris in the laser-exposed area. Thus, some plates withthe infrared-absorbing and other coating layers of the printing membersof this invention may typically also be imaged by laser ablation imagingfor use as wet lithographic plates. However, this alternative is notcommercially attractive compared to the much higher imaging speed andnon-ablative imaging process with no or extremely low airborne effluentsand excellent cleanability and image quality of the methods of thepresent invention.

Example 3

A three layer lithographic printing plate in accordance with theinvention, as illustrated in FIG. 4, was prepared by applying an inkaccepting layer of the following composition on a dry weight basis to anExample 2D plate.

Ink Accepting Surface Layer Composition 6 sec Nitrocellulose polymer1.94 CYMEL 303 0.74 NACURE 2530 (as PTSA) 0.35 The 6 secondNitrocellulose was obtained from Aldrich Chemical Company as 70% blendwith isopropanol.

The above components were mixed with 1-methoxy-2-propanol to give a 3%solids solution. An Example 2D plate was coated with this solution usinga #3 wire wound rod and dried in a hot air oven for one minute at 175°F.

This three layer plate was exposed on a Creo TRENDSETTER 1.7X at 140 RPMand 14 watts (exposure of 280 mj/cm²) with a GATF test image. Verylittle odor was detected coming from the plate surface and the image inthe laser-exposed areas was only weakly visible with very slight loosedebris seen when wiped with a dry finger. The exposed plate was easilycleaned by scrubbing with water using a series of molleton rolls.

The non-image background areas showed no residual infrared-absorbingcoating and very fine highlight features were visible. The image areashad excellent durability. Two percent 150 lpi halftone dots and 0.05point lines were fully retained after 50 vigorous double rubs with awater wet WEBRIL, and solid areas showed negligible change after 50vigorous double rubs with a water wet WEBRIL wipe or 50 vigorous doublerubs with a methyl ethyl ketone wet WEBRIL wipe. Water beaded up onimage areas.

While the invention has been described in detail and with reference tospecific and general embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A method of imaging a wet positive workinglithographic printing member, said method comprising the steps of: (a)providing a positive-working lithographic printing member, said membercomprising a hydrophilic metal substrate, a hydrophilic layer overlyingsaid substrate, and an ink-accepting surface layer overlying saidhydrophilic layer, wherein (i) said surface layer is characterized byabsorption of infrared imaging radiation and by being not removable bycleaning with water or a cleaning solution prior to said absorption ofinfrared imaging radiation, and (ii) said hydrophilic layer ischaracterized by being unremovable by cleaning with said water orcleaning solution; (b) exposing, in an imagewise pattern, said member toabsorbable infrared radiation to effect absorption thereof by saidsurface layer, thereby causing said surface layer in said laser-exposedareas to become removable by cleaning with said water or cleaningsolution; and (c) removing, with said water or cleaning solution, saidlaser-exposed areas of said surface layer to reveal the underlyinghydrophilic layer, wherein said surface layer comprises one or morepolymers and an infrared-absorbing carbon black sensitizer present insaid surface layer in an amount greater than 55% by weight.
 2. A methodof imaging a wet positive working lithographic printing member, saidmethod comprising the steps of: (a) providing a positive-workinglithographic printing member, said member comprising a hydrophilic metalsubstrate, a hydrophilic layer overlying said substrate, and anink-accepting surface layer overlying said hydrophilic layer, wherein(i) said surface layer is characterized by absorption of infraredimaging radiation and by being not removable by cleaning with water or acleaning solution prior to said absorption of infrared imagingradiation, and (ii) said hydrophilic layer is characterized by beingunremovable by cleaning with said water or cleaning solution; (b)exposing, in an imagewise pattern, said member to absorbable infraredradiation to effect absorption thereof by said surface layer, therebycausing said surface layer in said laser-exposed areas to becomeremovable by cleaning with said water or cleaning solution; and (c)removing, with said water or cleaning solution, said laser-exposed areasof said surface layer to reveal the underlying hydrophilic layer,wherein said surface layer comprises one or more polymers and aninfrared-absorbing carbon sensitizer present in an amount greater than65% by weight of said surface layer.
 3. A method of imaging a wetpositive working lithographic printing member, said method comprisingthe steps of: (a) providing a positive-working lithographic printingmember, said member comprising a substrate, a hydrophilic layeroverlying said substrate, and an ink-accepting surface layer overlyingsaid hydrophilic layer, wherein (i) said surface layer comprises apolyvinyl alcohol and is characterized by absorption of infrared imagingradiation and by being not removable by cleaning with water or acleaning solution prior to said absorption of infrared imagingradiation, and (ii) said hydrophilic layer is characterized by beingunremovable by cleaning with said water or cleaning solution; (b)exposing, in an imagewise pattern, said member to absorbable infraredradiation to effect absorption thereof by said surface layer, therebycausing said surface layer in said laser-exposed areas to becomeremovable by cleaning with said water or cleaning solution; and (c)removing, with said water or cleaning solution, said laser-exposed areasof said surface layer to reveal the underlying hydrophilic layer.
 4. Themethod of claim 3, wherein said polyvinyl alcohol is present in anamount of 20 to 95% by weight of a total polymer weight of said surfacelayer.
 5. The method of claim 3, wherein said polyvinyl alcohol ispresent in an amount of 25 to 75% by weight of a total polymer weight ofsaid surface layer.
 6. A method of imaging a wet positive workinglithographic printing member, said method comprising the steps of: (a)providing a positive-working lithographic printing member, said membercomprising a substrate, a hydrophilic layer overlying said substrate,and an ink-accepting surface layer overlying said hydrophilic layer,wherein (i) said surface layer comprises one or more polymers, acrosslinking agent, a catalyst and an infrared-absorbing sensitizer andis characterized by absorption of infrared imaging radiation and bybeing not removable by cleaning with water or a cleaning solution priorto said absorption of infrared imaging radiation, and (ii) saidhydrophilic layer is characterized by being unremovable by cleaning withsaid water or cleaning solution; (b) exposing, in an imagewise pattern,said member to absorbable infrared radiation to effect absorptionthereof by said surface layer, thereby causing said surface layer insaid laser-exposed areas to become removable by cleaning with said wateror cleaning solution; and (c) removing, with said water or cleaningsolution, said laser-exposed areas of said surface layer to reveal theunderlying hydrophilic layer.
 7. The method of claim 6, wherein saidcatalyst comprises an organic sulfonic acid component.
 8. The method ofclaim 7, wherein said organic sulfonic acid component is a component ofan amine-blocked p-toluenesulfonic acid.
 9. The method of claim 7,wherein said organic sulfonic acid component is present in an amount of25 to 75% by weight of a total polymer weight of said surface layer. 10.The method of claim 7, wherein said organic sulfonic acid component ispresent in an amount of 35 to 55% by weight of a total polymer weight ofsaid surface layer.
 11. The method of claim 7, wherein said surfacelayer comprises greater than 5% by weight of said organic sulfonic acidcomponent.
 12. The method of claim 7, wherein said surface layercomprises greater than 12% by weight of said organic sulfonic acidcomponent.
 13. A method of imaging a wet positive working lithographicprinting member, said method comprising the steps of: (a) providing apositive-working lithographic printing member, said member comprising asubstrate, a hydrophilic layer overlying said substrate, and anink-accepting surface layer overlying said hydrophilic layer, wherein(i) said surface layer is characterized by absorption of infraredimaging radiation and by being not removable by cleaning with water or acleaning solution prior to said absorption of infrared imagingradiation, and (ii) said hydrophilic layer is characterized by beingunremovable by cleaning with said water or cleaning solution and saidhydrophilic layer comprises a crosslinked, polymeric reaction product ofa hydrophilic polymer and a first crosslinking agent that is a zirconiumcompound; (b) exposing, in an imagewise pattern, said member toabsorbable infrared radiation to effect absorption thereof by saidsurface layer, thereby causing said surface layer in said laser-exposedareas to become removable by cleaning with said water or cleaningsolution; and (c) removing, with said water or cleaning solution, saidlaser-exposed areas of said surface layer to reveal the underlyinghydrophilic layer.
 14. The method of claim 13, wherein said firstcrosslinking agent is ammonium zirconyl carbonate.
 15. A method ofimaging a wet positive working lithographic printing member, said methodcomprising the steps of: (a) providing a positive-working lithographicprinting member, said member comprising a substrate, a hydrophilic layeroverlying said substrate, and an ink-accepting surface layer overlyingsaid hydrophilic layer, wherein (i) said surface layer is characterizedby absorption of infrared imaging radiation and by being not removableby cleaning with water or a cleaning solution prior to said absorptionof infrared imaging radiation, and (ii) said hydrophilic layer ischaracterized by being unremovable by cleaning with said water orcleaning solution and comprises a crosslinked, polymeric reactionproduct of a polyvinyl alcohol and a first crosslinking agent that isammonium zirconyl carbonate, and further wherein said ammonium zirconylcarbonate is present in an amount greater than 10% by weight of saidpolyvinyl alcohol; (b) exposing, in an imagewise pattern, said member toabsorbable infrared radiation to effect absorption thereof by saidsurface layer, thereby causing said surface layer in said laser-exposedareas to become removable by cleaning with said water or cleaningsolution; and (c) removing, with said water or cleaning solution, saidlaser-exposed areas of said surface layer to reveal the underlyinghydrophilic layer.
 16. The method of claim 15 wherein wherein saidammonium zirconyl carbonate is present in an amount of 20 to 50% byweight of said polyvinyl alcohol.
 17. The method of claim 15, whereinsaid hydrophilic layer further comprises a second crosslinking agent.18. The method of claim 17, wherein said hydrophilic layer furthercomprises a crosslinked, polymeric reaction product of a polyvinylalcohol and said second crosslinking agent.
 19. The method of claim 18,wherein said second crosslinking agent is a melamine.
 20. The method ofclaim 17, wherein said hydrophilic layer further comprises a catalystfor said second crosslinking agent.
 21. The method of claim 20, whereinsaid catalyst is an organic sulfonic acid component.
 22. A method ofpreparing a wet lithographic printing member, said method comprising thesteps of: (a) coating onto a substrate a liquid mixture comprising afirst liquid medium, a hydrophilic polymer, and a first crosslinkingagent; (b) drying the layer formed in step (a) to remove said firstliquid medium, and to form a hydrophilic layer; (c) coating onto saidhydrophilic layer a liquid mixture comprising a second liquid medium, apolymer, an infrared-absorbing sensitizer, and a second crosslinkingagent; (d) drying the layer formed in step (c) to remove said secondliquid medium, and to form an ink-accepting surface layer, therebyforming a positive-working lithographic printing member, said surfacelayer and said hydrophilic layer being unremovable by cleaning withwater or a cleaning solution; (e) exposing, in an imagewise pattern,said member to absorbable infrared radiation to effect absorption bysaid surface layer, thereby causing said surface layer in saidlaser-exposed areas to become removable by cleaning with said water orcleaning solution; and (f) removing, with said water or cleaningsolution, said laser-exposed areas of said surface layer to reveal theunderlying hydrophilic layer.
 23. The method of claim 22 wherein dryingof the layer formed in step (a) causes a portion of the firstcrosslinking agent to react, and further wherein drying of the layerformed in step (c) causes an additional portion of said firstcrosslinking agent present in said hydrophilic layer to react and causesa portion of the second crosslinking agent present to react.
 24. Themethod of claim 23, wherein said hydrophilic layer is characterized bythe absence of removal of said hydrophilic layer in said laser-exposedareas during steps (e) and (f).
 25. The method of claim 23, wherein saidabsorption of infrared radiation in the laser-exposed areas of saidsurface layer of step (e) is sufficient to cause said surface layer insaid laser-exposed areas to become removable by cleaning with said wateror cleaning solution but insufficient to remove by ablation greater than5% by weight of the surface layer in said laser-exposed areas.
 26. Themethod of claim 23, wherein said absorption of infrared radiation in thelaser-exposed areas of said surface layer of step (e) is sufficient tocause said surface layer in said laser-exposed areas to become removableby cleaning with said water or cleaning solution but insufficient toremove by ablation greater than 2% by weight of the surface layer insaid laser-exposed areas.
 27. The method of claim 23, wherein saidabsorption of infrared radiation in the laser-exposed areas of saidsurface layer of step (e) is sufficient to cause said surface layer insaid laser-exposed areas to become removable by cleaning with said wateror cleaning solution but insufficient to remove by ablation any surfacelayer in said laser-exposed areas.
 28. The method of claim 23, whereinthe weight of said surface layer of step (d) is from 0.05 to 1.0 g/m².29. The method of claim 23, wherein the weight of said surface layer ofstep (d) is from 0.1 to 0.5 g/m².
 30. The method of claim 22 wherein aportion of said second crosslinking agent penetrates into saidhydrophilic layer and wherein the step of drying the layer formed instep (c) further comprises drying the underlying hydrophilic layer. 31.The method of claim 30, wherein said hydrophilic layer is characterizedby the absence of removal of said hydrophilic layer in saidlaser-exposed areas during steps (e) and (f).
 32. The method of claim30, wherein said absorption of infrared radiation in the laser-exposedareas of said surface layer of step (e) is sufficient to cause saidsurface layer in said laser-exposed areas to become removable bycleaning with water or said cleaning solution but insufficient to removeby ablation greater than 5% by weight of the surface layer in saidlaser-exposed areas.
 33. The method of claim 30, wherein said absorptionof infrared radiation in the laser-exposed areas of said surface layerof step (e) is sufficient to cause said surface layer in saidlaser-exposed areas to become removable by cleaning with said water orcleaning solution but insufficient to remove by ablation greater than 2%by weight of the surface layer in said laser-exposed areas.
 34. Themethod of claim 30, wherein said absorption of infrared radiation in thelaser-exposed areas of said surface layer of step (e) is sufficient tocause said surface layer in said laser-exposed areas to become removableby cleaning with said water or cleaning solution but insufficient toremove by ablation any surface layer in said laser-exposed areas. 35.The method of claim 30, wherein the weight of said surface layer of step(d) is from 0.05 to 1.0 g/m².
 36. The method of claim 30, wherein theweight of said surface layer of step (d) is from 0.1 to 0.5 g/m². 37.The method of claim 22 wherein (a) the liquid mixture coated onto thesubstrate comprises one or more hydrophilic polymers, said firstcrosslinking agent being present in an amount greater than 10% by weightof said one or more hydrophilic polymers; (b) the liquid mixture coatedonto said hydrophilic layer comprises one or more polymers; and (c) saidsensitizer is present in an amount of 25 to 80% by weight of saidsurface layer, and said one or more polymers are present in an amount of10 to 60% by weight of said surface layer.
 38. The method of claim 37,wherein said hydrophilic layer is characterized by the absence ofremoval of said hydrophilic layer in said laser-exposed areas duringsteps (e) and (f).
 39. The method of claim 37, wherein said absorptionof infrared radiation in the laser-exposed areas of said surface layerof step (e) is sufficient to cause said surface layer in saidlaser-exposed areas to become removable by cleaning with said water orcleaning solution but insufficient to remove by ablation greater than 5%by weight of the surface layer in said laser-exposed areas.
 40. Themethod of claim 37, wherein said absorption of infrared radiation in thelaser-exposed areas of said surface layer of step (e) is sufficient tocause said surface layer in said laser-exposed areas to become removableby cleaning with said water or cleaning solution but insufficient toremove by ablation greater than 2% by weight of the surface layer insaid laser-exposed areas.
 41. The method of claim 37, wherein saidabsorption of infrared radiation in the laser-exposed areas of saidsurface layer of step (e) is sufficient to cause said surface layer insaid laser-exposed areas to become removable by cleaning with said wateror cleaning solution but insufficient to remove by ablation any surfacelayer in said laser-exposed areas.
 42. The method of claim 37, whereinthe weight of said surface layer of step (d) is from 0.05 to 1.0 g/m².43. The method of claim 37, wherein the weight of said surface layer ofstep (d) is from 0.1 to 0.5 g/m².
 44. The method of claim 37, whereinone of said one or more polymers of step (c) comprises a polymerselected from the group consisting of: polyvinyl alcohol, polyurethanes,epoxy polymers, vinyl polymers, acrylic polymers, and cellulosics.
 45. Amethod of imaging a wet positive-working lithographic printing member,said method comprising the steps of: (a) providing a positive-workinglithographic printing member, said member comprising a substrate, ahydrophilic layer overlying said substrate, an ink-accepting surfacelayer overlying said hydrophilic layer, and a primer layer interposedbetween said hydrophilic layer and said surface layer; wherein (i) saidsurface layer is characterized by absorption of infrared imagingradiation, by being not removable by cleaning with water or a cleaningsolution prior to said absorption of infrared imaging radiation, and bybeing adapted to form a wet lithographic printing surface as a result ofan imagewise exposure to absorbable infrared radiation and subsequentremoval of the exposed areas of said surface layer by cleaning with saidwater or cleaning solution to reveal the underlying hydrophilic layer;(ii) said primer layer comprises a zirconium compound and anadhesion-promoting agent; and (iii) said hydrophilic layer ischaracterized by being unremovable by cleaning with said water orcleaning solution; (b) exposing, in an imagewise pattern, said member toabsorbable infrared radiation to effect absorption thereof by saidsurface layer, thereby causing said surface layer in said laser-exposedareas to become removable by cleaning with said water or cleaningsolution; and (c) removing, with said water or cleaning solution, saidlaser-exposed areas of said surface layer to reveal the underlyinghydrophilic layer.
 46. A method of preparing a wet lithographic printingmember, said method comprising the steps of: (a) coating onto asubstrate a liquid mixture comprising a first liquid medium, ahydrophilic polymer, and a first crosslinking agent; (b) drying thelayer formed in step (a) to remove said first liquid medium, to cause aportion of said first crosslinking agent present to react, and to form ahydrophilic layer; (c) coating onto said hydrophilic layer a liquidmixture comprising a second liquid medium and an adhesion-promotingagent; (d) drying the layer formed in step (c) to remove said secondliquid medium and to form a primer layer; (e) coating onto said primerlayer a liquid mixture comprising a third liquid medium, a polymer, aninfrared-absorbing sensitizer, and a second crosslinking agent; (f)drying the layer formed in step (e) to remove said third liquid medium,to cause an additional portion of said first crosslinking agent presentin said hydrophilic layer to react, to cause a portion of said secondcrosslinking agent present to react, and to form an ink-acceptingsurface layer; thereby forming a positive-working lithographic printingmember, wherein said surface layer and said hydrophilic layer arecharacterized by being not removable by cleaning with water or acleaning solution; (g) exposing said member of step (f) to absorbableinfrared radiation using an infrared-emitting laser to effect absorptionof infrared radiation in the laser-exposed areas of said surface layerthat is sufficient to cause said surface layer in said laser-exposedareas to become removable by cleaning with said water or cleaningsolution but insufficient to remove by ablation greater than 10% byweight of the surface layer in said laser-exposed areas; and (h)removing, with said water or cleaning solution, said laser-exposed areasof said surface layer to reveal the underlying hydrophilic layer.
 47. Amethod of preparing a wet lithographic printing member, said methodcomprising the steps of: (a) coating onto a substrate a liquid mixturecomprising a first liquid medium, a hydrophilic polymer, and a firstcrosslinking agent; (b) drying the layer formed in step (a) to removesaid first liquid medium and to form a hydrophilic layer; (c) coatingonto said hydrophilic layer a liquid mixture comprising a second liquidmedium and an adhesion-promoting agent; (d) drying the layer formed instep (c) to remove said second liquid medium and to form a primer layer;(e) coating onto said primer layer a liquid mixture comprising a thirdliquid medium, a polymer, an infrared-absorbing sensitizer, and a secondcrosslinking agent; wherein a portion of said second crosslinking agentpenetrates into said hydrophilic and primer layers; (f) drying the layerformed in step (e) and the underlying hydrophilic and primer layers toremove said third liquid medium, to cause a portion of said secondcrosslinking agent present in said hydrophilic layer to react, and toform an ink-accepting surface layer; thereby forming a positive-workinglithographic printing member, wherein said surface layer and saidhydrophilic layer are characterized by being not removable by cleaningwith water or a cleaning solution; (g) exposing said member of step (f)to absorbable infrared radiation using an infrared-emitting laser toeffect absorption of infrared radiation in the laser-exposed areas ofsaid surface layer that is sufficient to cause said surface layer insaid laser-exposed areas to become removable by cleaning with said wateror cleaning solution but insufficient to remove by ablation greater than10% by weight of the infrared-absorbing layer in said laser-exposedareas; and (h) removing, with said water or cleaning solution, saidlaser-exposed areas of said surface layer to reveal the underlyinghydrophilic layer.